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Soil aggregates store most soil organic carbon (SOC), but how does litter quality influence their formation? We hypothesized varying litter quality to facilitate differences in aggregate formation by altering the seasonal development of microbial biomass (MB) C and N, with MB driving aggregate development in a tropical sandy soil in Thailand. Aggregate development was studied in a long-term fallow experiment, receiving 10 Mg ha−1 annual applications of rice (Oryza sativa) straw (low N and polyphenols (PP)), groundnut (Arachis hypogaea) stover (high N, low PP), tamarind (Tamarindus indica) litter (medium N and PP), or dipterocarp (Dipterocarpus tuberculatus) leaf litter (low N, high PP) compared to a control. N-rich litter from groundnut and tamarind led to significantly higher MB, bulk soil C and aggregate C than dipterocarp, rice straw, and the control. Bulk soil C and small macroaggregates C of N-rich litter treatments increased about 7% in 30 weeks. Increasing MB N explained increasing small macroaggregate C and both, MB C or N were important covariates explaining temporal variations of C stored in themicroaggregates, in silt and clay. MB also explained temporal variations of aggregate fraction weights. With time, SMA C only increased in the N-rich groundnut and tamarind treatments, but decreased in other treatments. Connections of MB to aggregate C and weight substantiated the importance of microbial activity for aggregate formation and carbon sequestration. By promoting MB for longest time spans, medium-quality tamarind could best facilitateaggregate formation, and increase silt and clay C.

Brachiaria humidicola (syn. Urochloa humidicola) has been acknowledged to control soil nitrification through release of nitrification inhibitors (NI), a phenomenon conceptualized as biological nitrification inhibition (BNI). Liming and N fertilization as features of agricultural intensification may suppress BNI performance, due to a decrease in NI exudation, increased NH3 availability and promotion of ammonia oxidizing bacteria (AOB) over archaea (AOA). A 2-year three-factorial pot trial was conducted to investigate the influence of soil pH and soil microbial background (ratio of archaea to bacteria) on BNI performance of B. humidicola. The study verified the capacity of B. humidicola to reduce net nitrification rates by 50 to 85% compared to the non-planted control, irrespective of soil pH and microbial background. The reduction of net nitrification, however, was largely dependent on microbial N immobilization and efficient plant N uptake. A reduction of gross nitrification could not be confirmed for the AOA dominated soil, but possibly contributed to reduced net nitrification rates in the AOB-dominated soil. However, this putative reduction of gross nitrification was attributed to plant-facilitated inter-microbial competition between bacterial heterotrophs and nitrifiers rather than BNI. It was concluded that BNI may play a dominant role in extensive B. humidicola pasture systems, while N immobilization and efficient plant N uptake may display the dominant factors controlling net nitrification rates under intensively managed B. humidicola.

We present a comprehensive, high-quality dataset characterizing soil–vegetation and land surface processes from continuous measurements conducted in two climatically contrasting study regions in southwestern Germany: the warmer and drier Kraichgau region with a mean temperature of 9.7 ∘C and annual precipitation of 890 mm and the cooler and wetter Swabian Alb with mean temperature 7.5 ∘C and annual precipitation of 1042 mm. In each region, measurements were conducted over a time period of nine cropping seasons from 2009 to 2018. The backbone of the investigation was formed by six eddy-covariance (EC) stations which measured fluxes of water, energy and carbon dioxide between the land surface and the atmosphere at half-hourly resolution. This resulted in a dataset containing measurements from a total of 54 site years containing observations with a multitude of crops, as well as considerable variation in local growing-season climates. The presented multi-site, multi-year dataset is composed of crop-related data on phenological development stages, canopy height, leaf area index, vegetative and generative biomass, and their respective carbon and nitrogen content. Time series of soil temperature and soil water content were monitored with 30 min resolution at various points in the soil profile, including ground heat fluxes. Moreover, more than 1200 soil samples were taken to study changes of carbon and nitrogen contents. The dataset is available at https://doi.org/10.20387/bonares-a0qc-46jc (Weber et al., 2021). One field in each region is still fully set up as continuous observatories for state variables and fluxes in intensively managed agricultural fields.

Site‐specific soil fertility management requires a fundamental understanding of factors that modulate soil fertility variability in the local context. To verify this assumption, this study hypothesized that soil fertility variability across two regions in Central and Western Ethiopia is determined by inter‐related effects of agro‐ecological zones and farmers’ resource endowment (‘wealthy’ versus ‘poor’ farmers). Mid‐infrared spectroscopy coupled to partial least squares regression (midDRIFTS‐PLSR) and wet‐laboratory analyses were used to assess the soil fertility (soil pH, total soil carbon [TC] and nitrogen [TN], plant‐available phosphorous [Pav] and potassium [Kav]) across four agro‐ecological zones: ‘High‐Dega’ (HD), ‘Dega’ (D), ‘Weina‐Dega’ (WD) and ‘Kola’ (K). MidDRIFTS peak area analysis of spectral frequencies (2,930 [aliphatic C‐H], 1,620 [aromatic C = C], 1,159 [C‐O poly‐alcoholic and ether groups] cm‐1) was applied to characterize soil organic carbon (SOC) quality and to calculate the SOC stability index (1,620:2,930). Higher TC in HD, as well as higher TN and Kav contents in K were found in fields of wealthy compared with poor farmers. Resource endowment dependent soil fertility management options revealed SOC of higher quality in wealthy compared with poor farms in D. Agro‐ecological zones distinctions contributed to these soil fertility differences. Farmers distinguished visually fertile and less fertile fields based on soil colour. Higher pH in K and WD as well as Pav in K and HD were found in fertile (brown/black) than less fertile (red) soils. To conclude, tailor‐made soil fertility management in the local context must consider agro‐ecological zones and resource endowment interactions along with farmers’ indigenous knowledge., German Federal Ministry for Economic Cooperation and Development (BMZ) through the International Institute for Tropical Agricultural (IITA) with its LegumeCHOICE project

Biological Nitrification Inhibition (BNI) of Brachiaria humidicola has been attributed to nitrification-inhibiting fusicoccanes, most prominently 3-epi-brachialactone. However, its release mechanism from B. humidicola roots remains elusive. Two hydroponic experiments were performed to investigate the role of rhizosphere pH and nutritional N form in regulating 3-epi-brachialactone release by B. humidicola and verify the underlying release pathway. Low rhizosphere pH and NH4 + nutrition promoted 3-epi-brachialactone exudation. However, the substitution of NH4 + by K+ revealed that the NH4 + effect was not founded in a direct physiological response to the N form but was related to the cation-anion balance during nutrient uptake. Release of 3-epi-brachialactone correlated with the transmembrane proton gradient ΔpH and NH4 + uptake (R2 = 0.92 for high ~ 6.8 and R2 = 0.84 for low ~ 4.2 trap solution pH). This corroborated the release of 3-epi-brachialactone through secondary transport, with the proton motive force (ΔP) defining transport rates across the plasma membrane. It was concluded that 3-epi-brachialactone release cannot be conceptualized as a regulated response to soil pH or NH4 + availability, but merely as the result of associated changes in ΔP.

This study verified the inter‐related effect of ‘market distance’, defined as walking time, ‘farm typology’, defined as resource endowment, and ‘site’, defined as geographic location with contrasting agro‐ecologies, as well as farmers’ indigenous knowledge on soil fertility variability in smallholder farming systems in two distinct regions (Bushumba, Mushinga) of South‐Kivu, DR Congo. A total of 384 soil samples were selected from representative farmers’ fields and analysed for soil pH, soil organic carbon (SOC) content and quality, as well as nutrient contents, using midDRIFTS (mid‐infrared diffuse reflectance Fourier transform spectroscopy) and wet chemistry analyses. MidDRIFTS was also used to calculate SOC stability indexes as SOC quality proxies. ‘Market distance’ and ‘farm typology’ were key determinants of soil fertility variability, both with contrasting trends in Bushumba and Mushinga. Decreasing soil fertility with increasing market distance was noted across all farm typologies. ‘Farm typology’ was related to exchangeable calcium and magnesium, while ‘site’ resulted in a difference of plant available phosphorus. SOC quality indexes were related to ‘site’, interacting with ‘market distance’. A ‘market distance’ effect became obvious in the medium wealthy and poor farms of Mushinga, where a lower SOC quality in remote fields plots was noted with increasing market distance. In agreement with farmers’ indigenous knowledge, soil fertility levels were higher in deep than shallow soils, which were reflected in higher nutrient stocks in deep soils receiving organic amendments. Our results inferred that soil fertility variability across smallholder farms must consider various inter‐related determinants as basis for site‐specific fertility management interventions., German Federal Ministry for Economic Cooperation and Development (BMZ)

Worldwide, soil degradation due to erosion is still a major issue. Minimum tillage, intercropping, and alley cropping are important features of conservation agriculture and food security, but the farming communities are reluctant to accept these practices due to area reduction and plant competition. We investigated maize sole cropping (T1) as current farmers’ practice; T2: maize intercropped with chilies, tillage and fertilization; T3: maize-chili intercrop, mini. tillage, fertilization, Jack-bean relay cropping; T4: Like T3 but with hedges; T5 and T6 like T3 and T4, respectively, but without fertilization on Thai uplands during 2010 and 2011. Maize above ground biomass (AGB) was higher in T2 than T1 during both years. Light use efficiency (LUE) for AGB was 1.44–1.56 and 1.44–1.96 g DM MJ−1 in T4 during 2010 and 2011, 17–47% higher than T1, respectively. Land equivalent ratio was 1.23 and 1.17 in T2 while 1.28 and 1.21 in T4 during 2010 and 2011, respectively. Maize grain nitrogen (Ng) was significantly lower in unfertilized treatments. Intercropping with soil conservation enhanced land use efficiency with soil protection against erosion. Understanding spatial variations of LUE, Ng, as well as their impact on crop productivity are useful for fine-tuning crop management in conservation agriculture and sustainable land use on uplands.

Biological Nitrification Inhibition (BNI) of Brachiaria humidicola has been mainly attributed to the root-exuded fusicoccane-type diterpene brachialactone. We hypothesized, however, that according to the high diversity of fusicoccanes described for plants and microorganisms, BNI of B. humidicola is caused by an assemblage of bioactive fusicoccanes. B. humidicola root exudates were collected hydroponically and compounds isolated by semi-preparative HPLC. Chemical structures were revealed by spectroscopic techniques, including HRMS as well as 1D and 2D NMR. Nitrification inhibiting (NI) potential of isolated compounds was evaluated by a Nitrosomonas europaea based bioassay. Besides the previously described brachialactone (1), root exudates contained 3-epi-brachialactone (2), the C3-epimer of 1 (m/z 334), as well as 16-hydroxy-3-epi-brachialactone (3) with an additional hydroxyl group at C16 (m/z 350) and 3,18-epoxy-9-hydroxy-4,7-seco-brachialactone (4), which is a ring opened brachialactone derivative with a 3,18 epoxide ring and a hydroxyl group at C9 (m/z 332). The 3-epi-brachialactone (2) showed highest NI activity (ED50 ~ 20 μg mL−1, ED80 ~ 40 μg mL−1), followed by compound 4 with intermediate (ED50 ~ 40 μg mL−1), brachialactone (1) with low and compound 3 without activity. In coherence with previous reports on fusicoccanes, stereochemistry at C3 was of high relevance for the biological activity (NI potential) of brachialactones.

Soil organic matter (SOM) turnover models predict changes in SOM due to management and environmental factors. Their initialization remains challenging as partitioning of SOM into different hypothetical pools is intrinsically linked to model assumptions. Diffuse reflectance mid-infrared Fourier transform spectroscopy (DRIFTS) provides information on SOM quality and could yield a measurable pool-partitioning proxy for SOM. This study tested DRIFTS-derived SOM pool partitioning using the Daisy model. The DRIFTS stability index (DSI) of bulk soil samples was defined as the ratio of the area below the aliphatic absorption band (2930 cm−1) to the area below the aromatic–carboxylate absorption band (1620 cm−1). For pool partitioning, the DSI (2930 cm−1 ∕ 1620 cm−1) was set equal to the ratio of fast-cycling ∕ slow-cycling SOM. Performance was tested by simulating long-term bare fallow plots from the Bad Lauchstädt extreme farmyard manure experiment in Germany (Chernozem, 25 years), the Ultuna continuous soil organic matter field experiment in Sweden (Cambisol, 50 years), and 7 year duration bare fallow plots from the Kraichgau and Swabian Jura regions in southwest Germany (Luvisols). All experiments were at sites that were agricultural fields for centuries before fallow establishment, so classical theory would suggest that a steady state can be assumed for initializing SOM pools. Hence, steady-state and DSI initializations were compared, using two published parameter sets that differed in turnover rates and humification efficiency. Initialization using the DSI significantly reduced Daisy model error for total soil organic carbon and microbial carbon in cases where assuming a steady state had poor model performance. This was irrespective of the parameter set, but faster turnover performed better for all sites except for Bad Lauchstädt. These results suggest that soils, although under long-term agricultural use, were not necessarily at a steady state. In a next step, Bayesian-calibration-inferred best-fitting turnover rates for Daisy using the DSI were evaluated for each individual site or for all sites combined. Two approaches significantly reduced parameter uncertainty and equifinality in Bayesian calibrations: (1) adding physicochemical meaning with the DSI (for humification efficiency and slow SOM turnover) and (2) combining all sites (for all parameters). Individual-site-derived turnover rates were strongly site specific. The Bayesian calibration combining all sites suggested a potential for rapid SOM loss with 95 % credibility intervals for the slow SOM pools' half-life being 278 to 1095 years (highest probability density at 426 years). The credibility intervals of this study were consistent with several recently published Bayesian calibrations of similar two-pool SOM models, i.e., with turnover rates being faster than earlier model calibrations suggested; hence they likely underestimated potential SOM losses.

Organic matter management can improve soil structural properties. This is crucial for agricultural soils in tropical regions threatened by high rainfall intensities. Compared to conventional farming, organic farming is usually deemed to increase organic carbon and improve soil structural properties such as stability and permeability. However, how much, if any, buildup of organic carbon is possible or indeed occurring also depends on soil type and environmental factors. We compared the impact of seven years of organic farming (annually 13.6 t ha−1 of composted manure) with that of conventional practices (2 t ha−1 of farmyard manure with 150–170 kg N ha−1 of mineral fertilizers) on soil structural properties. The study was conducted on a Vertisol in India with a two‐year crop rotation of cotton soybean wheat. Despite large differences in organic amendment application, organic carbon was not significantly different at 9.6 mg C g−1 on average in the topsoil. However, the size distribution of water‐stable aggregates shifted toward more aggregates, Biovision Foundation for Ecological Development http://dx.doi.org/10.13039/501100015593, Coop Sustainability Fund, Swiss Agency for Development and Cooperation (SDC), Foundation fiat panis http://dx.doi.org/10.13039/501100011087, Liechtenstein Development Service http://dx.doi.org/10.13039/501100015698, https://doi.org/10.6084/m9.figshare.18665612

Increasing livestock densities on limited grazing areas in African savannahs lead to resource degradation through overgrazing, aggravated by drought. Assessing herd management strategies over longer periods at landscape scale is important to propose options for sustainable land use. This requires an understanding of processes related to hydrology, nutrient cycling, herd movement, pasture degradation, and animal resilience that involve dynamic soil-plant-animal interactions and human decisions about stocking rates, livestock purchases and sales. We present the coupled model system MPMAS-LUCIA-LIVSIM (MLL), the combination of a spatially explicit agent-based model for human decision-making (MPMAS), a spatially distributed landscape model for water flows, nutrient cycles and plant growth (LUCIA), and a herd model (LIVSIM) representing grazing, body weight, nutrition and excreta of individual animals. MLL represents daily vegetation growth in response to grazing and organic inputs, monthly animal performance influenced by forage availability and quality, and herders’ management in response to resource status. New modules for selective grazing, resprouting of pasture, herd movement and model coupling were developed for MLL. The test case of a pastoral system in the Ethiopian Borana region demonstrates the capabilities of MLL to simulate key soil-plant-animal-human interactions under climate-related management scenarios with varying access to grazing land, changing cattle prices and different spending / saving behaviour of herders. 20-year simulations showed the negative impact of consecutive drought years on vegetation biomass, on herd development and movement and how reserving grazing areas for dry seasons could mitigate overgrazing and improve income. Seasonality and drought response of vegetation growth, selective grazing of different plant parts, resprouting after grazing, calving intervals, milk yields and lactation in response to forage supply and quality as well as herder reactions to shocks were plausibly represented. Building upon this successful proof-of-concept, MLL can be used to identify robust management options for improved grazing systems in savannahs in follow-up research.

Intensifying grassland-based ruminant livestock systems (GRLS) in the African Guinea Savanna could contribute to food and income security of the increasing African population. Grassland-based ruminant livestock models (GRLM) may be useful to holistically assess the efficacy and sustainability of different intensification options. Therefore, we evaluated how adequately existing GRLM address future modelling needs required for assessing how sustainable and resilient different GRLS intensification options are relative to dynamic socio-economic, political, cultural, and environmental conditions. Key aspects future GRLM should thus address include, inter- and intra-species competition for resources, feeding behaviour of ruminant livestock and their adaptive capacity to changing environmental conditions, emissions from GRLS, and the role of policy on farmers’ decision-making. Current GRLM adequately simulate GRLS productivity, but only partly address proposed modelling needs. To improve these GRLM, coupling to or integration in modelling tools focusing on neglected issues and research on interrelationships between diverse GRLS components are suggested.

It was hypothesized that the application of eucalyptus biochar enhances nutrient use efficiencies of simultaneously supplied fertilizer, as well as provides additional nutrients (i.e., Ca, P, and K), to support crop performance and residual effects on subsequent crops in a degraded sandy soil. To test this hypothesis, we conducted an on‐farm field experiment in the Khon Kaen province of Northeastern Thailand to assess the effects of different application rates of eucalyptus biochar in combination with mineral fertilizers to upland rice and a succeeding crop of sugarcane on a sandy soil. The field experiment consisted of three treatments: (1) no biochar; (2) 3.1 Mg ha⁻¹ biochar (10.4 kg N ha⁻¹, 3.1 kg P ha⁻¹, 11.0 kg K ha⁻¹, and 17.7 kg Ca ha⁻¹); (3) 6.2 Mg ha⁻¹ biochar (20.8 kg N ha⁻¹, 6.2 kg P ha⁻¹, 22.0 kg K ha⁻¹, and 35.4 kg Ca ha⁻¹). All treatments received the same recommended fertilizer rate (32 kg N ha⁻¹, 14 kg P ha⁻¹, and 16 kg K ha⁻¹ for upland rice; 119 kg N ha⁻¹, 21 kg P ha⁻¹, and 39 kg K ha⁻¹ for sugarcane). At crop harvests, yield and nutrient contents and nitrogen (N) use efficiency were determined, and soil chemical properties and pH₀ monitored. The eucalyptus biochar material increased soil Ca availability (117 ± 28 and 116 ± 7 mg kg⁻¹ with 3.1 and 6.2 Mg ha⁻¹ biochar application, respectively) compared to 71 ± 13 mg kg⁻¹ without biochar application, thus promoting Ca uptake and total plant biomass in upland rice. Moreover, the higher rate of eucalyptus biochar improved CEC, organic matter, available P, and exchangeable K at succeeding sugarcane harvest. Additionally, 6.2 Mg ha⁻¹ biochar significantly increased sugarcane yield (41%) and N uptake (70%), thus enhancing N use efficiency (118%) by higher P (96%) and K (128%) uptake, although the sugar content was not increased. Hence, the application rate of 6.2 Mg ha⁻¹ eucalyptus biochar could become a potential practice to enhance not only the nutrient status of crops and soils, but also crop productivity within an upland rice–sugarcane rotation system established on tropical low fertility sandy soils.

On low-input smallholder farms of Kenyan upland landscapes, erosion of nutrient-rich topsoil strongly affects crop yields. Where maize (Zea mays) is intercropped on erosion-prone slopes, intercropping can potentially reduce soil erosion. The objective of this research was to quantify the contribution of crops and crop mixtures of different growth habits to erosion control and their influence on above-ground biomass and earthworm abundance as indicators of soil function in smallholder farming systems under a bimodal rainfall pattern in Western Kenya. The experiment involved five treatments, namely maize (Z. mays)/common bean (Phaseolus vulgaris) intercrop (maize intercrop), maize/common bean intercrop plus Calliandra (Calliandra calothyrsus) hedgerows and Calliandra mulch (Calliandra), sole Lablab (Lablab purpureus), sole Mucuna (Mucuna pruriens) and groundnut (Arachis hypogaea) intercropped with maize (during the short rains). The experiment was conducted over three consecutive cropping seasons and the cropping system had significant effects on soil loss, runoff, water infiltration, earthworm abundance and above-ground biomass and crop grain yield. The Calliandra treatment had the lowest runoff (11.6–17.2 mm ha−1) and soil erosion (31–446 kg ha−1 per season) in all the seasons, followed by the Mucuna treatment. Lablab was affected by disease and showed the highest soil erosion in the last two seasons. Infiltration was highest in Calliandra treatment, and earthworm abundance was higher under Mucuna and Calliandra treatments (229 and 165 earthworms per square metre, respectively) than under other crops. Our results suggest that including sole crops of herbaceous species such as Mucuna, or tree hedgerows with mixtures of maize and grain legumes has the potential to reduce runoff and soil erosion in smallholder farming. Additionally, these species provide a suitable habitat for earthworms which stabilise soil structure and macropores and thus potentially increase infiltration, further reducing soil erosion.

Against the background of current understanding of dissolved organic carbon (DOC) adsorption onto clay surfaces, it remains unclear if bulk DOC or its fractions contribute to microaggregate formation in the top layers of coarse‐textured soils. We therefore investigated the effects of long‐term inputs of biochemically contrasting organic residues on the chemical characteristics and vertical distribution of DOC in a coarse‐textured Ultisol. During 2007–2008, DOC samples were extracted from soil profiles of a long‐term residue quality field experiment initiated in 1995. In this field experiment, groundnut stover, dipterocarp and tamarind leaf litter, as well as rice straw of contrasting biochemical quality, were applied yearly at 10 Mg ha⁻¹. Groundnut, dipterocarp and tamarind produced large amounts (7.1–11.8 g C m⁻²) of high‐molecular‐weight (HMW; > 10 kDa) DOC, which was found in high concentrations (30–50 mg C kg⁻¹) in the topsoil (0–15 cm). Rice straw, however, produced large amounts (3.5 g C m⁻²) of low‐molecular‐weight (LMW; < 1 kDa) DOC during the initial stage of decomposition. Although the HMW DOC was retained in the topsoil (0–15 cm), the LMW DOC was rapidly translocated to lower soil depths (60–80 cm). This translocation was facilitated by the low adsorption potential of the rice straw‐derived LMW DOC on colloidal surfaces of the topsoil. There was a significant positive correlation of C in the HMW DOC with that in fine particles, indicating their contribution to microaggregate formation and thus C accumulation. It was concluded that biochemical quality of residues as a determinant of concentration and chemistry of DOC and its vertical dynamics along the soil profile must be considered for SOC accumulation in coarse‐textured soils. Furthermore, we found reasonable indications that HMW DOC contributes to microaggregate formation in topsoils. HIGHLIGHTS: Residue quality determined vertical dynamics of DOC in coarse– textured Ultisols. Lignin‐ and polyphenol‐rich residues produced HMW DOC in topsoil. LMW DOC derived from cellulose‐rich residues was translocated to the subsoil. There were indications that HMW DOC supported microaggregate formation in topsoils.

Large‐scale conversion of natural forest to rubber plantations has taken place for decades in Southeast Asia, help to make it a deforestation hot spot. Besides negative changes in biodiversity, ecosystem water, and carbon budgets, converting forests to plantations often reduced CH₄ uptake by soils. The latter process, which might be partly responsible for resumed increase in the growth rate of CH₄ atmospheric concentrations since 2006, has not been adequately investigated. We measured soil surface CH₄ fluxes during 2014 and 2015 in natural forests and rubber plantations of different age and soil textures in Xishuangbanna, Southwest China—a representative area for this type of land‐use change. Natural forest soils were stronger CH₄ sinks than rubber soils, with annual CH₄ fluxes of −2.41 ± 0.28 and −1.01 ± 0.23 kg C ha⁻¹ yr⁻¹, respectively. Water‐filled pore space was the main factor explaining the differences between natural forests and rubber plantations, even reverting rubber soils temporarily from CH₄ sink into a methane source during the rainy season in older plantations. Soil nitrate content was often lower under rubber plantations. Added as a model covariate, this factor improved explanation power of the CH₄ flux—water‐filled pore space regression. Although soils under rubber plantation were more clayey than soils under natural forest, this was not the decisive factor driving higher soil moisture and lower CH₄ uptake in rubber soils. Thus, the conversion of forests into rubber plantations exerts a negative impact on the CH₄ balance in the tropics and likely contributes to global climate.

Less attention has been devoted to the understanding of the sequential effect of biological nitrogen fixation (BNF) on microbial ammonia (NH3) oxidation in terrestrial soils. To shed light into this ecologically important relationship, a 15N2-DNA-stable isotope probing (SIP) technique was applied to explore the incorporation of 15N fixed by BNF into the genome of NH3 oxidizers in planted and non-planted flooded rice soils, using the α-subunit of NH3 monooxygenase (amoA gene) as a functional marker. The abundance and structural composition of archaeal and bacterial amoA genes in density-resolved fractions obtained from SIP were investigated by quantitative PCR, as well as terminal-restriction fragment length polymorphism and sequence analyses. Results revealed that bacterial rather than archaeal NH3 oxidizers assimilated 15N derived from BNF in both planted and non-planted flooded rice soils. The amoA genes in 15N-enriched “heavy” SIP gradient fractions were closely related to K-strategic Nitrosospira cluster 10, including Nitrosospira sp. AF-like bacteria, suggesting an ecological linkage of these two metabolically distinct processes. It remains unsolved from our study why exactly Nitrosospira cluster 10 was capitalizing on BNF. It could be suggested that K-strategy is the underlying ecological mechanism of sustainable, self-regulating terrestrial ecosystems such as traditional, unfertilized flooded rice soils. In this regard, it needs to be assayed if some rice-associated diazotrophs might also be classified as K-strategists, since BNF is an energy demanding metabolic process.

Biological nitrogen (N) fixation (BNF) plays a significant role in maintaining soil fertility in paddy field ecosystems. Rice variety influences BNF, but how different rice varieties regulate BNF and associated diazotroph communities has not been quantified. Airtight, field-based 15N2-labelling growth chamber experiments were used to assess the BNF capacity of different rice varieties. In addition, both the 16S rRNA and nifH genes were sequenced to assess the influence of different rice varieties on bacterial and diazotrophic communities in paddy soils. After subjecting a rice-soil system to 74 d of continuous airtight, field-based 15N2 labelling in pots in a growth chamber, the amounts of fixed N were 22.3 and 38.9 kg ha–1 in inbred japonica (W23) and hybrid indica (IIY) rice cultivars planted in the rice-soil systems, respectively, and only 1%–2.5% of the fixed N was allocated to the rice plants and weeds. A greater abundance of diazotrophs was found in the surface soil (0–1 cm) under IIY than under W23. Sequencing of the 16S rRNA gene showed significantly greater abundances of the cyanobacterial genera Nostoc, Anabaena, and Cylindrospermum under IIY than under W23. Sequencing of the nifH gene also showed a significantly greater abundance of Nostoc under IIY than under W23. These results indicate that the hybrid rice cultivar (IIY) promoted BNF to a greater extent than the inbred rice cultivar (W23) and that the increase in BNF might have been due to the enhanced heterocystous cyanobacteria Nostoc.

Organic farming has been proposed as a solution to foster agricultural sustainability and mitigate the negative environmental impacts of conventional farming. This study assessed N losses and soil surface N balances in conventional and organic farming systems in a sub-humid and semi-humid (Chuka and Thika) sites in Kenya. Nitrate–N (NO3−–N) leached was trapped at 1 m depth using the Self Integrating Accumulator core method and the changes in mineral-N were assessed at different soil depths and different crop growth stages. Both conventional and organic farming systems lost substantial amounts of NO3−–N at the early growth stages of all the crops. Cumulative NO3−–N leached was similar in all the farming systems in each cropping season. More NO3−–N was leached during potato cropping (22–38 kg N ha−1) than during maize (0.9–5.7 kg N ha−1) and vegetable cropping (1.9–2.9 kg N ha−1). Under maize cultivation, three times more NO3−–N was leached at Chuka site than at Thika site. During the potato cropping, between 79 and 83% of the N applied in the low input systems was leached, compared to 10–20% in the high input systems. Only Org-High exhibited a positive soil surface N balance (797–1263 kg ha−1) over a whole rotation period at both sites. We recommend reducing N applications for potato in all farming systems and at the early growth stages for all the crops in order to reduce N loss to the environment. We also recommend increasing N application rates in the low input systems and to developing a model to guide application of organic inputs.

Para rubber (Hevea brasiliensis Mull. Arg) plantations have expanded into regions with sub-optimal growth conditions: distinct dry seasons and temperatures cooler than in humid tropics. The impact of these new marginal environments and future climate change on rubber tree development and latex yield is largely unknown. This hampers reliable prediction of farmers’ revenues and extent of carbon sequestration at landscape level. To improve our understanding of rubber trees response to planting at high altitudes and associated increase in planting densities, we applied the process-based Land Use Change Impact Assessment tool (LUCIA). It was calibrated with detailed ground survey data from Xishuangbanna, southwest China to model tree biomass development and latex yield in rubber plantations at the tree, plot and landscape level. Plantations were analyzed at 600 trees ha−1) were tested. Four greenhouse gas emission scenarios, with Representative Concentration Pathways (RCP) ranging from the lowest RCP 2.6 to the highest emission scenario RCP 8.5, were used to test rubber tree response to climate change. During a 40-year rotation under current climate, lowland rubber plantations grew faster and had larger latex yields than highland rubber. The average biomass of lowland rubber was 9% and 18% higher than those of highland rubber for aboveground and belowground biomass, respectively. High planting density rubber plantations showed 5% and 4% higher above ground biomass than those at low- and medium-planting density, but simulations suggest that the cumulative latex production decreased strongly by 26% and 14% respectively. The results of the RCP 8.5 climate change scenario suggested that during 40 years simulation mean total biomass and cumulative latex yield of highland rubber (per tree) increased by 28% and 48%, while lowland rubber increased by 8% and 10% respectively when compared to the baseline. Other rubber cultivation regions could also benefit from this modelling approach that helps in optimization of carbon stock and latex production in rubber-based system. The results could help in development of future climate change adaption and mitigation strategies.

To match Nitrogen (N) supply to crop N demand, it is essential to understand N release and uptake patterns in different farming systems and crops. To assesses the dynamics of N released in organic and conventional systems and potential synchrony and asynchrony in crop N uptake, a study was conducted over three cropping seasons (potato, maize and leafy vegetables) at two sites in the Central Highlands of Kenya. Mineral-N release and synchrony were monitored in conventional and organic systems at high (recommended N, P, pesticides and irrigation) and low input (low N, P, pesticide use and rainfed) systems. Mineral-N release was assessed using in situ buried bags and N synchrony was measured by the daily differences in N fluxes. The percentage of N applied released during potato (38%) and vegetable (44%) cropping seasons were similar between systems. However, under maize strong temporal N immobilization from inputs occurred, particularly at Thika, related to the poor quality of manure and compost (lignin:N ratio > 13). In all systems, excess-asynchrony of available N was pronounced during vegetative stages and at harvest, while insufficient-asynchrony occurred at reproductive stages. During potato cropping season at Thika, Org-High showed highest positive N fluxes (> 20 kg N ha−1 day−1) at planting and tuber bulking stage. At early stages of maize and vegetables Org-Low and Org-High experienced up to 5 times larger negative N fluxes (insufficiency) compared to conventional treatments at Chuka site. The study recommends reducing N applications at planting and increasing N dosages at reproductive stages of crops.

Soil, inputs, and environmental factors such as weather control plant nutrient availability and nutrient content in food. Drought periods affect nutrient bioavailability. Nutrient transport within the plant and allocation of nutrients within organs of the plant is water dependent and therefore drought susceptible. This study compared Kapchorwa, Uganda and Teso South, Kenya that experienced drought during the second season in 2016. The main research questions were: (i) do droughts have an impact on the nutrient composition of food; (ii) is there a difference in nutrient concentrations in food based on their xylem or phloem mobility? Maize (Zea mays) grain (n = 62) and matooke (Musa acuminata) fruit samples (n = 90) in Kapchorwa, and maize grain (n = 61) and cassava (Manihot esculenta) tuber (n = 64) in Teso South were collected during a normal season (March–July) and drought season (October–December) in 2016. Crop samples were analysed using a pXRF for P, K, Ca, Mg, S, Fe, Mn, Cu, and Zn. The Standardized Precipitation Index (SPI) was calculated using TAMSAT database to compare drought intensities. The drought in Kapchorwa (SPI: −1.14 to −0.32) was severer and began 2 months prior to Teso South (SPI: 0.09 to 0.55). Nutrient concentration in Kapchorwa decreased significantly from normal to drought in both crops. In contrast, during the moderate drought in Teso South, nutrient concentrations increased significantly. Lacking nutrient phloem mobility is suggested to play a vital role in mobilisation of micronutrients (Fe, Mn, and Cu) as shown by their decreased concentration under severe drought in the yield. Total nutrients assimilated in crop samples were significantly higher in the normal than the drought for almost all samples. Micronutrients and yields during drought were strongly affected, leading to a double-burden for consumers through affected quantity and quality. Future research considerations should particularly include the focus on potential nutrient increases during mild drought.

Rubber farming expansion in Xishuangbanna, in the Upper Mekong region of Southwest China, has resulted in profound land use change and led to the severe degradation of the local environment. This study explores the dynamics of land use change as a result of the rubber boom, examines the factors influencing the heterogeneity in farmers' land allocations for rubber farming, and assesses the implications of this change for the local environment in terms of carbon balances. The analyses use a comprehensive household survey data set of 612 smallholder rubber farmers in Xishuangbanna. The historical data illustrate the trajectory of rubber expansion and land use change over the past three decades. The model of smallholder land allocation for rubber farming suggests its determinants include ethnicity, experience in rubber farming, household wealth, elevation, and several explanatory variables at the village level. A net loss in carbon stocks at the aggregate level was found due to the expansion of rubber plantations. The rubber farming expansion of smallholders outside the natural reserves in Xishuangbanna has led to a carbon loss of approximately 21 Mg/ha/year over the past three decades. The findings complement discussions on the future of the rubber-based land use system and its sustainability in Xishuangbanna and other rubber-growing areas in the Mekong region.