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Straw incorporation is crucial to soil organic carbon (SOC) sequestration, thus improving soil fertility and mitigating climate change. The fate of straw C and the associated net SOC balance remain largely unexplored, particularly in soils subjected to long-term mineral and organic fertilization. To address this, soil (δ13C: –19‰) that had been continuously cropped with maize for 31 years and subjected to five long-term fertilization regimes, including (i) control (Unfertilized), (ii) mineral fertilizer (NPK) application, (iii) 200% NPK (2 × NPK) application, (iv) manure (M) application, and (v) NPK plus manure (NPKM) application, was incubated with or without addition of rice straw (δ13C: –29‰) for 70 days. Straw addition largely primed SOC mineralization. The priming effect (PE) was considerably higher in 2 × NPK (+122% of CO2 from soil without straw addition) but lower in M (+43%) relative to the unfertilized soil (+82%), highlighting the importance of fertilization in controlling PE intensity. Fertilization increased the straw-derived microbial biomass C by 90–577% and straw-derived SOC by 34–68% compared to the unfertilized soil, primarily due to the increased abundance of Gram-negative bacteria and cellobiohydrolase activity. Straw-derived SOC was strongly positively correlated with straw-derived microbial biomass C, suggesting that dead microbial biomass (necromass) was a dominant precursor of SOC formation. Consequently, fertilization facilitated microbial utilization of straw C and its retention in soil, particularly in the M and NPKM fertilized soils. The amounts of straw-derived SOC overcompensated for the SOC losses by mineralization, resulting in net C sequestration which was highest in the NPK fertilized soil. Our study emphasizes that NPK fertilization decreases the intensity of the PE induced by straw addition and increases straw C incorporation into SOC, thus facilitating C sequestration in agricultural soils.

Liming of acidic soils has been suggested as a strategy to enhance N2O reduction to N2 during heterotrophic denitrification, and mitigate N2O emission from N fertilised soils. However, the mechanisms involved and possible interactions of key soil parameters (NO3− and O2) still need to be clarified. To explore to what extent soil pH controls N2O emissions and the associated N2O/(N2O + N2) product ratio in an acidic sandy soil, we set-up three sequential incubation experiments using an unlimed control (pH 4.1) and a limed soil (pH 6.9) collected from a 50-year liming experiment. Interactions between different NO3− concentrations, N forms (ammonium- and nitrate) and oxygen levels (oxic and anoxic) on the liming effect of N2O emission and reduction were tested in these two sandy soils via direct N2 and N2O measurements. Our results showed 50-year liming caused a significant increase in denitrification and soil respiration rate of the acidic sandy soil. High concentrations of NO3− in soil (>10 mM N in soil solution, equivalent to 44.9 mg N kg−1 soil) almost completely inhibited N2O reduction to N2 (>90%) regardless of the soil pH value. With decreasing NO3− application rate, N2O reduction rate increased in both soils with the effect being more pronounced in the limed soil. Complete N2O reduction to N2 in the low pH sandy soil was also observed when soil NO3− concentration decreased below 0.2 mM NO3−. Furthermore, liming evidently increased both N2O emissions and the N2O/(N2+N2O) product ratio under oxic conditions when supplied with ammonium-based fertiliser, possibly due to the coupled impact of stimulated nitrification and denitrification. Overall, our data suggest that long-term liming has the potential to both increase and decrease N2O emissions, depending on the soil NO3− level, with high soil NO3− levels overriding the assumed direct pH effect on N2O/(N2+N2O) product ratio.

The return of agricultural crop residues are vital to maintain or even enhance soil fertility. However, the influence of application rate of crop residues on denitrification and its related gaseous N emissions is not fully understood. We conducted a fully robotized continuous flow incubation experiment using a Helium/Oxygen atmosphere over 30 days to examine the effect of maize straw application rate on: i) the rate of denitrification, ii) denitrification product stoichiometry N2O/(N2O+N2), and iii) the contribution of fungal denitrification to N2O fluxes. Five treatments were established using sieved, repacked sandy textured soil; i) non-amended control, ii) nitrate only, iii) low rate of straw + nitrate, iv) medium rate of straw + nitrate, and iv) high rate of straw + nitrate (n = 3). We simultaneously measured NO, N2O as well as direct N2 emissions and used the N2O 15N site preference signatures of soil-emitted N2O to distinguish N2O production from fungal and bacterial denitrification. Uniquely, soil NO3− measurements were also made throughout the incubation. Emissions of N2O during the initial phase of the experiment (0–13 days) increased almost linearly with increasing rate of straw incorporation and with (almost) no N2 production. However, the rate of straw amendment was negatively correlated with N2O, but positively correlated with N2 fluxes later in the experimental period (13–30 days). Soil NO3− content, in all treatments, was identified as the main factor responsible for the shift from N2O production to N2O reduction. Straw amendment immediately lowered the proportion of N2O from bacterial denitrification, thus implying that more of the N2O emitted was derived from fungi (18 ± 0.7% in control and up to 40 ± 3.0% in high straw treatments during the first 13 days). However, after day 15 when soil NO3− content decreased to

The winter wheat–summer maize rotation system in the North China Plain is a major source of nitrous oxide (N2O) emissions due to high nitrogen (N) fertilizer and irrigation water inputs. However, a detailed understanding of the contribution of N2O production sources is still limited because of the complexity of N2O generation in soils and a lack of relevant field studies. Moreover, the efficiency and mechanisms of N2O mitigation approaches in this area, i.e. the use of nitrification inhibitors, remains poorly understood. To elucidate the N2O production pathways from this rotation system and to evaluate the effect of a widely used nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on mitigating N2O emissions, we monitored N2O fluxes and analyzed isotopomer ratios of soil-emitted N2O during one rotation year. Results indicate that the application of DMPP significantly reduced N2O emissions by 67% in the winter wheat season and 47% in the summer maize season. Isotopomer analysis revealed that in the N-fertilized treatment, nitrification and/or fungal denitrification accounted for up to 36% of the N2O emission peaks observed after fertilization and irrigation events, whereas the nitrifier denitrification pathway was likely to be the major source, accounting for the remaining N2O emissions. The high effectiveness of the nitrification inhibitor on mitigating N2O emissions at high soil moisture may be attributed to the dual inhibitory effect on nitrifier denitrification, i.e. reducing the supply of nitrite, which is the substrate of nitrifier denitrification and inhibiting ammonia-oxidizing bacteria activities, which carry nitrifier denitrification.

Due to their extreme aridity, high rate of UV irradiation and low soil carbon (C) content, the soils of the Atacama Desert represent one of the world's most hostile environments for microbial life and its survival. Although infrequent, climatic conditions may, however, prevail which temporarily remove these stresses and allow life to briefly flourish. In this study we investigated the response of soil microbial communities to water and C availability across an aridity gradient (semi-arid, arid, hyper-arid) within the Atacama Desert. We simulated the impact of hyper-dry spells, humid fogs and precipitation events on the activation of the microbial community and the subsequent mineralization of low (glucose) and high (plant residues) molecular weight C substrates. Our results showed that mineralization rate followed the trend: semi-arid > arid > hyper-arid. Some glucose mineralization was apparent under hyper-arid conditions (water activity, aw = 0.05), although this was 10-fold slower than under humid conditions and ca. 200-fold slower than under wet conditions. A lag phase in CO2 production after glucose-C addition in the hyper-arid soils suggested that mineralization was limited by the low microbial biomass in these soils. No lag phase was apparent in the corresponding semi-arid or arid soils. In contrast, the breakdown of the plant residues was initially much slower than for glucose and involved a much longer lag phase in all soils, suggesting that mineralization was limited by low exoenzyme activity, particularly in the humid and hyper-dry soils. Our results also showed that microbial C use efficiency followed the trend: hyper-arid > arid > semi-arid. In conclusion, we have shown that even under hyper-arid conditions, very low levels of microbial activity and C turnover do occur. Further, the microbial communities are capable of rapidly responding to available C once water becomes more abundant, however, this response is both biomass and metabolically limited in hyper-arid soils.

The hyperarid soils of the Atacama Desert, Chile, contain the largest known nitrate deposits in the world. They also represent one of the most hostile environments for microbial life anywhere in the terrestrial biosphere. Despite known for its extreme dryness, several heavy rainfall events causing localised flash flooding have struck Atacama Desert core regions during the last five years. It remains unclear, however, whether these soils can support microbial denitrification. To answer this, we sampled soils along a hyperaridity gradient in the Atacama Desert and conducted incubation experiments using a robotized continuous flow system under a He/O2 atmosphere. The impacts of four successive extreme weather events on soil-borne N2O and N2 emissions were investigated, i) water addition, ii) NO3− addition, iii) labile carbon (C) addition, and iv) oxygen depletion. The 15N–N2O site-preference (SP) approach was further used to examine the source of N2O produced. Extremely low N2O fluxes were detected shortly after water and NO3− addition, whereas pronounced N2O and N2 emissions were recorded after labile-C (glucose) amendment in all soils. Under anoxia, N2 emissions increased drastically while N2O emissions decreased concomitantly, indicating the potential for complete denitrification at all sites. Although increasing aridity significantly reduced soil bacterial richness, microbial potential for denitrification and associated gene abundance (i.e., napA, narG, nirS, nirK, cnorB, qnorB and nosZ) was not affected. The N2O15N site preference values based on two end-member model suggested that fungal and bacterial denitrification co-contributed to N2O production in less arid sites, whereas bacterial denitrification dominated with increasing aridity. We conclude that soil denitrification functionality is preserved even with lowered microbial richness in the extreme hyperarid Atacama Desert. Future changes in land-use or extreme climate events therefore have a potential to destabilize the immense reserves of nitrate and induce significant N2O losses in the region.

Microbial transformation of crop residue is the key process of soil organic matter (SOM) formation and mineralization, which determines soil fertility and affects global climate change. However, utilization dynamics of residue-derived carbon (residue C) by various microbial communities is still not well understood, especially under different residue quality and soil fertility conditions over a long-term scale (i.e., >1 year). In this study, a 500-day in-situ field experiment was conducted using 13C-labeled maize (Zea mays L.) root and shoot (composed of both stem and leaf) to examine the role of microbial community composition on the C processing. Specifically, the mineralization of residue C and incorporation of residue C into microbial biomass in low fertility (LF) and high fertility (HF) soils were investigated. The abundance of 13C in soil samples and microbial phospholipid fatty acids (PLFAs) were measured after 60, 90, 150 and 500 days since the residues added. The results showed that the mineralization rate of residue C was significantly higher in the LF than that in the HF soil for the first 150 days, and the shoot-derived C was more susceptible to degradation than root-derived C, but the final mineralization rates (~78%) were not significantly different among treatments on the day 500. Soil fertility significantly affected the relative composition of different microbial groups and distribution of residue C in microbial communities, but residue type did not do so. Furthermore, residue C contributed more to PLFA-C pool in the LF than HF treatments, and the proportion of root C in PLFA-C pool was higher than that of shoot C, indicating easier immobilization of root C by soil microbial anabolism. Accordingly, soil fertility and residue quality could both regulate the kinetics of the microbial immobilization of crop residue C, but overall the available residual quantity of applied (plant-derived) C to enhance or maintain soil C pool did not depend on them in a long term in the agricultural Alfisols.

Assessing effects of organic fertilizer applications on N 2 O emissions is of great interest because they can cause higher N 2 O emissions compared to inorganic fertilizers for a given amount of added nitrogen (N). But there are also reports about enhanced N 2 O reduction to climate-neutral elemental N 2 after application of organic manures to soils. Factors controlling the N 2 O/(N 2 O + N 2 ) product ratio of denitrification are interrelated, and also the ratio is difficult to study because of limitations in N 2 flux measurements. In this study, we investigated N 2 O and N 2 emissions from soil treated with organic fertilizers with different C/N ratios. An N 2 O isotopomer approach combined with conventional N 2 O and N 2 flux measurements was employed to study underlying microbial pathways. A grassland soil was amended with anaerobic digestate (AD) from food waste digestion (low C/N ratio) or cattle slurry (CS; high C/N ratio), respectively, adjusted to 90% WFPS, and incubated for 52 days under helium–oxygen atmosphere (10% O 2 ) using a soil incubation system capable of automated N 2 O, N 2 , and CO 2 measurements. N 2 O isotopomer signatures, i.e. the δ 18 O and SP values (site preference between 15 N at the central and the peripheral position in the N 2 O molecule), were determined by Isotope Ratio Mass Spectrometry and used to model and subsequently estimate the contribution of bacterial denitrification and autotrophic nitrification to N 2 O production. For this approach the direct determination of emitted N 2 is essential to take isotope effects during N 2 O reduction to N 2 into account by correcting the measured isotope signatures for isotope effects during N 2 O reduction using previously determined fractionation factor ranges. The addition of both organic fertilizers to soil drastically increased the rate of gaseous N emissions (N 2 O + N 2 ), probably due to the effects of concurrent presence of nitrate and labile C on the denitrification rate. In the initial phase of the experiment (day 1 to ∼15), gaseous N emissions were dominated by N 2 fluxes in soils amended with organic manures; meanwhile, N 2 O emissions were lower compared to untreated Control soils, but increased after 15–20 days relative to the initial fluxes, especially with CS. Extremely low N 2 O, but high N 2 emissions in the initial phase suggest that reduction of N 2 O to N 2 via denitrification was triggered when the soil was amended with organic fertilizers. In contrast in the untreated Control, N 2 O release was highest during the initial phase. Total N 2 O release from AD treated soil was similar to Control, while N 2 O from CS treated soil was considerably higher, indicating that denitrification was triggered more by the high labile carbon content in CS, while the cumulative N 2 O/(N 2 O + N 2 ) product ratio and thus N 2 O reduction were similar with both organic fertilizers. The results of the N 2 O source partitioning based on the isotopomer data suggest that about 8–25% (AD) and 33–43% (CS) of the cumulated N 2 O emission was due to nitrification in organically amended soil, while in the untreated Control nitrification accounted for about 5–16%. The remaining N 2 O production was attributed mainly to denitrification, while the poor model fit for other source pathways like fungal denitrification suggested their contribution to be of minor importance. The observed rather distinct phases with predominance first of denitrification and later of nitrification may help developing mitigation measures by addressing N 2 O source processes individually with appropriate management options. The observation of relatively large shares of nitrification-derived N 2 O is surprising, but may possibly be related to the low soil pH and will require further investigation. The determination of N 2 production is essential for this isotopomer-based source partitioning approach, but so far only applicable under laboratory conditions. The results of this study indicate that the combination of N 2 O δ 18 O and SP values is very useful in obtaining more robust source estimates as compared to using SP values alone.

Nitrous oxide (N2O) is one of the major greenhouse gases emitted from soils, where it is mainly produced by nitrification and denitrification. It is well known that rates of N2O release from soils are mainly determined by the availability of substrates and oxygen, but N2O source apportioning, highly needed to advance N2O mitigation strategies, still remains challenging. In this study, using an automated soil incubation system, the N2O site preference, i.e. the intramolecular 15N distribution, was analyzed to evaluate the progression in N2O source processes following organic soil amendment. Biogas fermentation residue (BGR; originating from food waste fermentation) was applied to repacked grassland soil cores and compared to ammonium sulfate (AS) application, both at rates equivalent to 160 kg NH4+–N ha−1, and to unamended soil (control). The soil cores were incubated in a helium–oxygen atmosphere with 20 kPa O2 for 43 days at 80% water-filled pore space. 43-day cumulative N2O emissions were highest with BGR treated soil accounting for about 1.68 kg N2O–N ha−1 while application of AS caused much lower fluxes of c. 0.23 kg N2O–N ha−1. Also, after BGR application, carbon dioxide (CO2) fluxes showed a pronounced initial peak with steep decline until day 21 whereas with ammonium addition they remained at the background level. N2O dual isotope and isotopomer analysis of gas samples collected from BGR treated soil indicated bacterial denitrification to be the main N2O generating process during the first three weeks when high CO2 fluxes signified high carbon availability. In contrast, in the second half after all added labile carbon substrates had been consumed, nitrification, i.e. the generation of N2O via oxidation of hydroxylamine, gained in importance reaching roughly the same N2O production rate compared to bacterial denitrification as indicated by N2O SP. Overall in this study, bacterial denitrification seemed to be the main N2O forming process after application of biogas residues and fluxes were mainly driven by available organic carbon.

We used dual labelled stable isotope (13C and 15N) techniques to examine how grassland plant species with different growth strategies vary in their ability to compete with soil microbes for different chemical forms of nitrogen (N), both inorganic and organic. We also tested whether some plant species might avoid competition by preferentially using different chemical forms of N than microbes. This was tested in a pot experiment where monocultures of five co-existing grassland species, namely the grasses Agrostis capillaris, Anthoxanthum odoratum, Nardus stricta, Deschampsia flexuosa and the herb Rumex acetosella, were grown in field soil from an acid semi-natural temperate grassland. Our data show that grassland plant species with different growth strategies are able to compete effectively with soil microbes for most N forms presented to them, including inorganic N and amino acids of varying complexity. Contrary to what has been found in strongly N limited ecosystems, we did not detect any differential uptake of N on the basis of chemical form, other than that shoot tissue of fast-growing plant species was more enriched in 15N from ammonium-nitrate and glycine, than from more complex amino acids. Shoot tissue of slow-growing species was equally enriched in 15N from all these N forms. However, all species tested, least preferred the most complex amino acid phenylalanine, which was preferentially used by soil microbes. We also found that while fast-growing plants took up more of the added N forms than slow-growing species, this variation was not related to differences in the ability of plants to compete with microbes for N forms, as hypothesised. On the contrary, we detected no difference in microbial biomass or microbial uptake of 15N between fast and slow-growing plant species, suggesting that plant traits that regulate nutrient capture, as opposed to plant species-specific interactions with soil microbes, are the main factor controlling variation in uptake of N by grassland plant species. Overall, our data provide insights into the interactions between plants and soil microbes that influence plant nitrogen use in grassland ecosystems.

Legumes increase the plant-available N pool in soil, but might also increase NO3− leaching to groundwater. To minimize NO3− leaching, N-release processes and the contribution of legumes to NO3− concentrations in soil must be known. Our objectives were (1) to quantify NO3−-N export to >0.3 m soil depth from three legume monocultures (Medicago x varia Martyn, Onobrychis viciifolia Scop., Lathyrus pratensis L.) and from three bare ground plots. Furthermore, we (2) tested if it is possible to apply a mixing model for NO3− in soil solution based on its dual isotope signals, and (3) estimated the contribution of legume mineralization to NO3− concentrations in soil solution under field conditions. We collected rainfall and soil solution at 0.3 m soil depth during 1 year, and determined NO3− concentrations and δ15N and δ18O of NO3− for >11.5 mg NO3−-N l−1. We incubated soil samples to assess potential N release by mineralization and determined δ15N and δ18O signals of NO3− derived from mineralization of non-leguminous and leguminous organic matter. Mean annual N export to >0.3 m soil depth was highest in bare ground plots (9.7 g NO3−-N m−2; the SD reflects the spatial variation) followed by Medicago x varia monoculture (6.0 g NO3−-N m−2). The O. viciifolia and L. pratensis monocultures had a much lower mean annual N export (0.5 and 0.3 g NO3−-N m−2). The averaged NO3−-N leaching during 70 days was not significantly different between field estimates and incubation for the Medicago x varia Martyn monoculture. The δ15N and δ18O values in NO3− of rainfall (δ15N: 3.3±0.8‰; δ18O: 30.8±4.7‰), mineralization of non-leguminous SOM (9.3±0.9‰; 6.7±0.8‰), and mineralization of leguminous SOM (1.5±0.6‰; 5.1±0.9‰) were markedly different. Applying a linear mixing model based on these three sources to δ15N and δ18O values in NO3− of soil solution during winter 2003, we calculated 18–41% to originate from rainfall, 38–57% from mineralization of non-leguminous SOM, and 18–40% from mineralization of leguminous SOM. Our results demonstrate that (1) even under legumes NO3−-N leaching was reduced compared to bare ground, (2) the application of a three-end-member mixing model for NO3− based on its dual isotope signals produced plausible results and suggests that under particular circumstances such models can be used to estimate the contributions of different NO3− sources in soil solution, and (3) in the 2nd year after establishment of legumes, they contributed approximately one-fourth to NO3−-N loss.

The mechanisms and specific sources of priming effects, i.e. short term changes of soil organic matter (SOM) decomposition after substance addition, are still not fully understood. These uncertainties are partly method related, i.e. until now only two C sources in released CO2 could be identified. We used a novel approach separating three carbon (C) sources in CO2 efflux from soil. The approach is based on combination of different substances originated from C3 or C4 plants in different treatments and identical transformation of substances like C3 sugar (from sugar beet) and C4 sugar (from sugar cane). We investigated the influence of the addition of two substances having different microbial utilizability, i.e. slurry and sugar on the SOM or/and slurry decomposition in two grassland soils with different levels of Corg (2.3 vs. 5.1% C). Application of slurry to the soil slightly accelerated the SOM decomposition. Addition of sugar lead to changes of SOM and slurry decomposition clearly characterized by two phases: immediately after sugar addition, the microorganisms switched from the decomposition of hardly utilizable SOM to the decomposition of easily utilizable sugar. This first phase was very short (2‐3 days), hence was frequently missed in other experiments. The second phase showed a slightly increased slurry and SOM decomposition (compared to the soil without sugar). The separation of three sources in CO2 efflux from grassland soils allowed us to conclude that the C will be utilized according to its utilizability: sugarOslurryOSOM. Additionally, decomposition of more inert C (here SOM) during the period of intensive sugar decomposition was strongly reduced (negative priming effect). We conclude that, priming effects involve a chain of mechanisms: (i) preferential substrate utilization, (ii) activation of microbial biomass by easily utilizable substrate (iii) subsequent increased utilization of following substrates according to their utilizability, and (iv) decline to initial state. q 2005 Elsevier Ltd. All rights reserved.

Sustainable agriculture requires the formation of new humus from the crops. We utilized 13C and 15N signatures of soil organic matter to assess how rapidly wheat/maize cropping contributed to the humus formation in coarse-textured savanna soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) cropped for lengths of time varying from 0 to 98 years, after conversion from native grassland savanna (C4). We performed natural 13C and 15N abundance measurements on bulk and particle-size fractions. The bulk soil δ13C values steadily decreased from −14.6 in (C4 dominated) grassland to −16.5‰ after 90 years of arable cropping. This δ13C shift was attributable to increasing replacement of savanna-derived C by wheat crop (C3) C which dominated over maize (C4) inputs. After calculating the annual C input from the crop yields and the output from literature data, by using a stepwise C replacement model, we were able to correct the soil δ13C data for the irregular maize inputs for a period of about one century. Within 90 years of cropping 41–89% of the remaining soil organic matter was crop-derived in the three studied agroecosystems. The surface soil C stocks after 90 years of the wheat/maize crop rotation could accurately be described with the Rothamsted Carbon Model, but modelled C inputs to the soil were very low. The coarse sand fraction reflected temporal fluctuations in 13C of the last C3 or C4 cropping and the silt fraction evidenced selective erosion loss of old savanna-derived C. Bulk soil 15N did not change with increasing cropping length. Decreasing δ15N values caused by fertilizer N inputs with prolonged arable cropping were only detected for the coarse sand fraction. This indicated that the present N fertilization was not retained in stable soil C pool. Clearly, conventional cropping practices on the South African highlands neither contribute to the preservation of old savanna C and N, nor the effective humus reformation by the crops.

Amino acids represent one of the largest inputs of dissolved organic nitrogen to soil and consequently they constitute a major component of the organic N cycle. The effect of agricultural management on the rate of amino acid turnover in soil, however, remains largely unknown. The aim of this study was to evaluate in long-term field experiments the effect of fertilizer addition (N, P and K), grazing, pH manipulation (lime addition), vegetation cover and shifts (grassland versus arable) and drainage on the mineralization of 14C-labelled amino acids in agricultural topsoils. Our results showed that the intrinsic rate of amino acid mineralization was rapid for all management regimes, irrespective of the tested soil type. The average (±SEM) half-life of the amino acids in all soils (n=155) was calculated to be 2.3±0.5 h. The relative amount of amino acid-C partitioned into respiration (25% of total C) versus biomass production (75% of total C) was also unaffected by management strategy. The rate of amino acid mineralization was shown to be slightly sensitive to soil pH, peaking at around pH ( CaCl 2 ) 5.0 with an approximate twofold reduction at the pH extremes (pH 3.8 and 6.4). We conclude that management regime has little effect on the intrinsic rate of amino acid mineralization in agricultural soils. We propose therefore that total microbial activity rather than microbial diversity or community structure is likely to be the key determinant governing amino acid turnover in agricultural soils.

Land application of animal wastes from intensive grassland farming has resulted in growing environmental problems relating to greenhouse gas emissions, ammonia volatilisation, and nitrate and phosphorus leaching into surface and groundwater. We examined the short-term effects of dairy slurry amendment on carbon sequestration and enzyme activities in a temperate grassland (Southwest England). Slurry was collected from cows fed either on perennial ryegrass (C 3 ) or maize (C 4 ) silages. Fifty m 3 ha −1 of each of the obtained C 3 or C 4 slurries ( δ 13 C=−30.7 and −21.3‰, respectively) were applied to a C 3 pasture soil with δ 13 C of −30.0±0.2‰. We found that water soluble organic carbon (WSOC) content was two to three times higher in the slurry amended plots compared with the unamended control. No significant change in the soil microbial biomass (SMB) carbon content was observed in the four weeks (772 h) following slurry application. Natural abundance 13 C isotope analysis suggested a rapid initial incorporation (>25% within 2 h of application) of slurry-derived C in the SMB-C and WSOC pools of the 0–2 cm layer. Linear relationships were found between slurry-derived C in the whole soil, SMB, and WSOC for the 0–2 cm depth in the soil. Applied slurry-derived C was sequestered in the SMB pool in two phases. The first phase (0–48 h) was dominated by the incorporation of labile slurry C from the liquid phase, whereas beyond 48 h slurry-derived C was mainly from less mobile particulate C. No significant differences between treatments were found for invertase and xylanase. Urease activity was always higher in slurry treatments. Cellobiohydrolase, β- N -acetyl-glucosamidase, β-glucosidase and acid phosphatase activities became significantly higher in slurry treatments after 336 h. However, the observed temporal changes in enzyme activities were not correlated with the amounts of slurry-C incorporated in the SMB and WSOC pool.

We investigated the specific diet and habitat of earthworms in relation to land use changes by integrating spatial and temporal scales and by using stable isotope (13C and 15N) techniques. The study involved two sites: Santiago (Northwest Spain) and North Wyke (Southwest England), both consisting of long term grassland which was partly converted to a maize crop in 1997. In 1998, the maize crop in Spain was divided into two, and one half was re-planted with maize (2 years maize) and the other half reconverted to grassland (1 year grassland); the same procedure was followed for the grassland resulting in two treatments, 2 years grassland and 1 year maize. At the English site only 2 years maize and 2 years grassland were under investigation. Within each of the four treatments in Spain and the two in England, three replicate plots were established. Random soil samples from three different depths (0–10, 10–20 and 20–30 cm), and earthworm specimens belonging to four different ecological categories (epigeic, anecic, epi/anecic and endogeic), were taken from each plot, treatment and site at the peak of the maize growth and after harvesting. Spanish soils were 13C-enriched and 15N-depleted when compared to the English ones, which was also reflected in the earthworm tissue, and allowed a direct relationship between the delta values of the animals and the cropping treatments. The enrichment in the 13C values of the worms feeding on the maize (C4) plots, when compared to those found under the grassland (C3) plots, was greater than the difference detected in the C4 vs C3 soils. This result clearly indicates selective feeding by earthworms with a preference for fresh C4 residue over older native C3. Different ecological and age groups appeared to consume organic material of differing quality, with endogeic species and mature worms showing the highest N isotope values as a result of preferential feeding in deeper soil profiles. This information proves that combined C and N isotope analysis constitutes a powerful tool in studying feeding ecology and emphasises the need for long-term studies which incorporate spatial and temporal scales to the experimental set-up.

To understand the role of dung-derived carbon in the carbon cycle of grazed temperate grasslands, we need a procedure to trace dung-derived C. The natural 13 C tracer technique of applying C4 dung to a C3 grass pasture allowed us to succesfully quantify the fate of cattle dung in the soil environment. Dung was collected from beef steers fed on either maize (C4) or perennial ryegrass (C3). The C4 dung ( δ 13 C −15.4‰ ) or C3 dung ( δ 13 C =−25.7‰ ) was applied in circular patches to a temperate (C3) grassland, with a bulk soil δ 13 C value of −27.9%. Triplicate samples were taken from 1–5, 5–10 and 10–20 cm depth in the soil, and from zero tension lysimeters (installed at 30 cm depth) at time intervals of 150 days following dung application. The soil and lysimeter solution samples ( δ 13 C and total C. Dung C was readily detectable in the upper 5 cm of the soil profile, but not below that depth. After 150 days, only 16.6% of the applied dung-C was accounted for, with 12.6% of dung C being recovered in the soil (1–5 cm depth) and 4.0% in leachate waters collected in lysimeters (installed at 30 cm depth). Apparently, only a minor proportion of dung C is retained in the grassland ecosystem.