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The strong affinity of phosphorus (P) to iron (Fe) oxides and hydroxides in highly weathered tropical soils limits P availability and therefore plant productivity in tropics. In flooded soils, however, P fixed by Fe oxides and hydroxides can be released into more available forms because of Fe 3 + reduction to Fe 2 + . These P dynamics in flooded soils are well documented for rice paddies. Such effects are much less studied in other land-use types influenced by seasonal flooding, especially in the tropics during heavy monsoon rains. The aim of this study was to investigate the P mobilization during flooding leading to anaerobic conditions in topsoil and subsoil depending on land-use type. Samples were collected in highly weathered Acrisols from four replicate sites under natural rainforest, jungle rubber, rubber and oil palm plantations in Sumatra, Indonesia. Topsoil and subsoil were taken to ensure a wide range of soil organic matter (SOM) and P contents. Soils were incubated under anaerobic, flooded conditions at 30 ± 1 °C for 60 days. Our results confirmed the hypothesis that soil flooding mobilizes P and increases P availability. Two distinct and opposite periods were observed during the flooding. During the first three weeks of flooding, the dissolved P (DP) concentration peaked, simultaneously with the peak of dissolved Fe 2 + (DFe 2 + ) and dissolved organic carbon (DOC). After three weeks, P availability in soils decreased, although Fe-P (P NaOH ) and available P (P NaHCO3 ) did not reach the initial, pre-flooding levels. The impacts of flooding on P and Fe forms was strong in the topsoil, where P dissolution and availability were generally higher under forest and, to a lesser extent, under jungle rubber. A positive correlation between DOC and DFe 2 + (R 2 = 0.42) in topsoil indicates that the intensity of microbially-mediated Fe 3 + reduction is limited by the amount of available carbon (C) as an energy source for microorganisms and as electron donor. Microbial mineralization of organic P from SOM also increases P availability, and this process requires available C. This interpretation was supported by the strong correlation (R 2 = 0.58) between available P and DOC, as well as between DP and DOC (R 2 = 0.56) in topsoil. The increasing pH in topsoil and subsoil after flooding of all land-use types may also influence the P release over time. In summary, the increase of available P and DP during flooding is due to three main mechanisms: (1) P release via the microbially-mediated reductive dissolution of Fe 3 + oxides; (2) P release during SOM mineralization and (3) solubility of Fe phosphate due to increasing pH. These mechanisms are relevant not only in riparian areas, where flooding occurs, but also in soils waterlogged after regular heavy rainfalls during the wet season. Therefore, we speculate that the P turnover is faster in compacted soils under plantations because of regular changes of oxic and anoxic conditions. Consequently, more P is pumped by the vegetation and then removed from plantations due to yield export. [ABSTRACT FROM AUTHOR]

Deforestation and land-use change in tropics have increased over the past decades, driven by the demand for agricultural products. Although phosphorus (P) is one of the main limiting nutrients for agricultural productivity in the tropics, the effect of land-use change on P availability remains unclear. The objective was to assess the impacts of land-use change on soil inorganic and organic P fractions of different availability (Hedley sequential fractionation) and on P stocks in highly weathered tropical soils. We compared the P availability under extensive land-use (rubber agroforest) and intensive land-use with moderate fertilization (rubber monoculture plantations) or high fertilization (oil palm monoculture plantations) in Indonesia. The P stock was dominated by inorganic forms (60 to 85%) in all land-use types. Fertilizer application increased easily-available inorganic P (i.e., H 2 O-Pi, NaHCO 3 -Pi) in intensive rubber and oil palm plantations compared to rubber agroforest. However, the easily-available organic P (NaHCO 3 -extractable Po) was reduced by half under oil palm and rubber. The decrease of moderately available and non-available P in monoculture plantation means that fertilization maintains only the short-term soil fertility that is not sustainable in the long run due to the depletion of P reserves. The mechanisms of this P reserve depletion are: 1) soil erosion (here assessed by C/P ratio), 2) mineralization of soil organic matter (SOM) and 3) P export with yield products. Easily-available P fractions (i.e., H 2 O-Pi, NaHCO 3 -Pi and Po) and total organic P were strongly positively correlated with carbon content, suggesting that SOM plays a key role in maintaining P availability. Ecologically based management is therefore necessary to mitigate SOM losses and thus increase the sustainability of agricultural production in P-limited, highly weathered tropical soils. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) decomposition in response to global change represents a critical uncertainty in coupled carbon (C) cycle-climate models. Much of this uncertainty arises from our limited mechanistic knowledge of the effects of organic C input frequency on SOM decomposition. Based on a three-source-partitioning isotopic approach (14C glucose addition to soil continuously labeled by C 4 plants over 21 years) and literature review of 86 observations, we assessed the priming of fast- and decadal-SOM decomposition after occasional and frequent (every 12 vs 60 days) labile C input in a grassland soil. Frequent glucose input accelerated SOM decomposition over 200 days, but the occasional input reduced this positive priming by 2.6 times. The positive priming by occasional C input resulted in 139 μg C g−1 soil net C sequestration corresponding to 38% of C input. During the 200 days incubation, the primed fast-cycling C (younger than 21 years) under occasional C addition was 52–94% greater compared with frequent addition. Conversely, the priming of decadal SOM (older than 21 years) by frequent C input was 463 μg C g−1 after the 200 days incubation, which represents 5.3% of the initial decadal-cycling soil C pool. This was 1.5 times higher than by occasional input because of the stronger N mining caused by the continuously high activity of microbial K strategists under frequent C input. Our global scale assessment showed that occasional C input reduces the priming of decadal SOM decomposition by 89–127 Tg per year compared to frequent input and thus, sequestrated an additional 102 Tg C per year. Overall, the vulnerability of decadal SOM to increasing C input frequency thus weakening the C sink of grassland, whilst reduced C input frequency (e.g. by drought) suppresses the SOM priming and reduces the positive feedback of SOM decomposition to global change. This points out the necessity to assess the age of soil C when predicting the consequences of altered soil C input frequency under global change. • Frequent C input induces stronger priming of the decadal-than fast-cycling C pool. • Microbial N mining and the dominant growth of K -strategy cause higher priming effect under frequent C input. • Occasional C input reduces the SOM priming by 1.5 times relative to frequent inputs. • The intensity of priming was increased with time under frequent C input. [ABSTRACT FROM AUTHOR]

Tropical regions, such as Sumatra, experiencing extensive transformation of natural ecosystems, are close to complete exhaustion of available land. Agroecosystems strongly modify water and nutrient cycles, leading to losses of soil fertility, C sequestration and biodiversity. Although large companies are the main drivers of deforestation and plantation establishment, smallholders account for 40% of the oil palm and the majority of the rubber production in Indonesia. Here, we assess the extent and mechanisms of soil degradation under smallholder oil palm and rubber plantations in a context of land scarcity. The topsoil properties (C and N contents, C stocks, C/N ratio, bulk density) in 207 oil palm and rubber plantations in the Jambi province of Sumatra were determined beside trees, inside rows and interrows. Soils under oil palms were on average more degraded than under rubber, showing lower C content and stocks, lower N and higher bulk density. While soil properties were homogenous under rubber, two opposite trends were observed under oil palm plantations: the majority of soils had C content <2.2%, but about one fifth of the plantations had >9% C. This resulted from the establishment of oil palms under conditions of land scarcity. Because the oil palm boom started when rubber was already well-established, oil palms were frequently planted in marginal areas, such as peatlands or riparian areas (high C) or soils degraded by previous use (low C). The management of oil palms led to subsequent soil degradation, especially in interrows: C content decreased and bulk density increased in older oil palm plantations. This was not observed in rubber plantations because of a C input from leaf litter spread homogeneously all over the plantation, higher ground cover and a limited use of motorized vehicles. Considering that 10% of soils under oil palms had very low C content (<1%), we conclude that intensive cultivation can lead to intensive soil degradation and expect future degradation of soils under young oil palms. This challenges the sustainability of agricultural intensification in Sumatra. Because Sumatra is a pioneer of tropical land-use change, this should be regarded as potential threats that other tropical regions may face in future. [ABSTRACT FROM AUTHOR]

Tropical forest conversion to agricultural land leads to a strong decrease of soil organic carbon (SOC) stocks. While the decrease of the soil C sequestration function is easy to measure, the impacts of SOC losses on soil fertility remain unclear. Especially the assessment of the sensitivity of other fertility indicators as related to ecosystem services suffers from a lack of clear methodology. We developed a new approach to assess the sensitivity of soil fertility indicators and tested it on biological and chemical soil properties affected by rainforest conversion to plantations. The approach is based on (non-)linear regressions between SOC losses and fertility indicators normalized to their level in a natural ecosystem. Biotic indicators (basal respiration, microbial biomass, acid phosphatase), labile SOC pools (dissolved organic carbon and light fraction) and nutrients (total N and available P) were measured in Ah horizons from rainforests, jungle rubber, rubber ( Hevea brasiliensis ) and oil palm ( Elaeis guineensis ) plantations located on Sumatra. The negative impact of land-use changes on all measured indicators increased in the following sequence: forest < jungle rubber < rubber < oil palm. The basal respiration, microbial biomass and nutrients were resistant to SOC losses, whereas the light fraction was lost stronger than SOC. Microbial C use efficiency was independent on land use. The resistance of C availability for microorganisms to SOC losses suggests that a decrease of SOC quality was partly compensated by litter input and a relative enrichment by nutrients. However, the relationship between the basal respiration and SOC was non-linear; i.e. negative impact on microbial activity strongly increased with SOC losses. Therefore, a small decrease of C content under oil palm compared to rubber plantations yielded a strong drop in microbial activity. Consequently, management practices mitigating SOC losses in oil palm plantations would strongly increase soil fertility and ecosystem stability. We conclude that the new approach enables quantitatively assessing the sensitivity and resistance of diverse soil functions to land-use changes and can thus be used to assess resilience of agroecosystems with various use intensities. [ABSTRACT FROM AUTHOR]

• The role of land-use change, crop rotation and site conditions were elucidate. • SOC was depleted in croplands compared to grasslands and mountain pastures. • SOC dynamics was more influenced by land-use changes than management practices. • P excess in soils enables C sequestration without additional P sequestration. • SOC balancing budget is a realistic approach to encourage C sequestration practices. Increasing soil organic carbon (SOC) in agroecosystems enables to address simultaneously multiple goals such as climate change adaptation and mitigation as well as food security. As croplands are depleted in SOC, they offer a great potential to sequester atmospheric carbon (C). Nonetheless, croplands are still losing SOC under most of the current agricultural systems. Although many factors driving SOC dynamics have already been identified, their relative importance has not been quantified yet. Using one of the densest European soil monitoring networks with 250 sites established in western Switzerland, in the present study we (i) assessed long-term (over 30 years) SOC dynamics in croplands (CR), permanent grasslands (PG) and mountain pastures (MP), and (ii) prioritized the importance of land use, soil characteristics and sites conditions in driving SOC dynamics. The SOC levels in PG and MP were similar when clay content was accounted for, whereas CR were depleted in SOC by 3.9 mg C mg−1 clay as compared to PG. The majority (61 %) of CR had SOC:clay ratio below 1:10, but only 16 % of PG and MP sites reached this threshold. By contrast, soil organic matter stoichiometry (C:N:Porg ratios) was similar in CR and PG for comparable SOC content. The increase of C:Porg ratio with SOC content (dilution effect) and the high total P in CR and PG (legacy effect) indicate the possibility to sequester atmospheric C at reduced nutrient sequestration costs. SOC changes ranged from -0.61 to 1.32 mg g-1 soil yr−1 and were the highest in sites that experienced land-use changes. No PG were losing SOC, while CR sites exhibited both SOC gains and losses. Because of the predominance of the initial SOC content on SOC dynamics, land-use history must be accounted for when assessing the effect of management practices. The main manageable factors driving SOC dynamics were the time under temporary or permanent grasslands along with the soil total P. As PG already are rich in SOC and total P, organic amendments should be partly redirected to CR. [ABSTRACT FROM AUTHOR]

• Flooding in riparian areas counters the effects of land use change on C storage. • C preservation due to oxygen-limited mineralization under anaerobic conditions. • δ13C values represent changing environmental conditions. Progressive conversion of tropical rainforests to agricultural monocultures in South East Asia increasingly affects landscape types such as riparian areas. The impacts of conversions on soil organic matter (SOM) vary with changing landforms. However, this was often not accounted for in previous studies where SOM in soils in riparian areas was combined with SOM from well-drained adjacent slopes. Because riparian areas have a high carbon (C) storage potential, our objectives were i) to assess their C stocks after conversion to rubber and oil palm plantations in Sumatra (Indonesia) and ii) to compare the impacts of land use conversion on C stocks between riparian and well-drained areas. Average soil C stock losses from the top 30 cm were about 14% and 4% following conversion of riparian forest to rubber and oil palm plantations, respectively, indicating a high resistance of C to mineralization. C losses from well-drained areas were twice as high as from riparian areas after the respective conversion. δ13C values from riparian areas showed clear heterogeneity down soil profiles that was explained i) by alternating oxic and anoxic conditions, leading to reduced SOM and litter decomposition in riparian areas and ii) by mineral sediments and organic matter deposition and accumulation by erosion from adjacent slopes covered by plantations. We conclude that riparian areas are more resilient in terms of soil C storage towards land-use change than well-drained areas because of sediment deposition and reduced oxygen availability. On this basis, we developed a conceptual model of the effects of land-use change and various ecotone characteristics on SOM mineralization in the top- and subsoil of riparian areas. [ABSTRACT FROM AUTHOR]

Abstract Severe constraints on grasslands productivity, ecosystem functions, goods and services are expected to result from projected warming and drought scenarios under climate change. Negative effects on vegetation can be mediated via soil fertility and water holding capacity, though specific mechanisms are fairly complex to generalise. In field drought experiments, it can be difficult to disentangle a drought effect per se from potential confounding effects related to vegetation or soil type, both varying along with climate. Furthermore, there is the need to distinguish the long-term responses of vegetation and soil to gradual climate shift from responses to extreme and stochastic climatic events. Here we address these limitations by means of a factorial experiment using a single dominant grassland species (the perennial ryegrass Lolium perenne L.) grown as a phytometer on two soils types with contrasted physicochemical characteristics, placed at two elevation sites along a climatic gradient, and exposed to early or late-season drought during the plant growing season. Warmer site conditions and reduced precipitation along the elevational gradient affected biogeochemistry and plant productivity more than the drought treatments alone, despite the similar magnitude in volumetric soil moisture reduction. Soil type, as defined here by its organic matter content (SOM), modulated the drought response in relation to local site climatic conditions and, through changes in microbial biomass and activity, determined the seasonal above and belowground productivity of L. perenne. More specifically, our combined uni- and multivariate analyses demonstrate that microbes in a loamy soil with low SOM are strongly responsive to change in climate, as indicated by a simultaneous increase in their C,N,P pools at high elevation with cooler temperatures and wetter soils. Contrastingly, microbes in a clay-loam soil with high SOM are mainly sensitive to temperature, as indicated by a strong increase in microbial biomass under warmer temperatures at low elevation and a concomitant increase in C:N, C:P and N:P ratios. High SOM promoted a better annual yield of the phytometer grass under warmer climate and the effect of drought on productivity was transient. In contrast, low SOM reduced cumulative yield under warmer conditions and root production strongly decreased, enduring a lasting drought effect. Microbes in soils with high organic matter remained more active during warmer and drier conditions, ensuring soil fertility and stimulating a higher overall plant nutrient availability and productivity. Our study highlights the important role of soil type for grassland responses to both stochastic climatic extremes and long-term climate change. Management practices enhancing SOM accumulation via organic residue incorporation seem a promising way to mitigate the effects of increased temperature and drought on plants and soil microbes alike promoting thereby a sustainable ecosystem functioning. Highlights • Warming-induced reduction in soil moisture affects soil fertility more than drought alone. • A soil with low SOM content is highly sensitive to change in precipitation regime. • A soil with high SOM content is sensitive to temperature. • High SOM increases plant yield in warmer conditions and drought effect is transient. • SOM accumulation mitigates the effect of increased temperature and drought on plants. [ABSTRACT FROM AUTHOR]