Dissolved organic nitrogen and carbon in pastoral soils: the New Zealand experience

Dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in soils are increasingly recognized as important components of nutrient cycling and biological processes in soil‐plant ecosystems. The aims of this study were to: (i) quantify the pools of DON and DOC in a range of New Zealand pastoral soils; (ii) compare the effects of land use changes on these pools; and (iii) examine the seasonal variability associated with these two components of dissolved organic matter. Soil samples (0–7.5 cm depth) from 93 pastoral sites located in Northland, Waikato, Bay of Plenty and Otago/Southland, New Zealand, were collected in autumn. Adjacent sites under long‐term arable cropping or native vegetation and forestry land use were also sampled at the same time to estimate the impacts of different land use on DON and DOC in these soils. Twelve dairy and 12 sheep and or beef pastures were sampled in winter, spring, summer and autumn for a 2‐year period to study the seasonal fluctuations of DON and DOC. A field incubation study was also carried out in a grazed pasture to examine fluctuations in the concentrations of and and DON levels in soil. Other soil biological properties, such as microbial biomass‐C, biomass‐N and mineralizable N, were also measured. Pastoral soils contained the greatest amounts of DON (13–93 mg N kg⁻¹ soil, equivalent to 8–55 kg N ha⁻¹) and DOC (73–718 mg C kg⁻¹ soil, equivalent to 44–431 kg C ha⁻¹), followed by cropping and native vegetation and forestry soils. The DON concentration in soils was found to be more seasonally variable than DOC. There was approximately 80% fluctuation in the concentration of DON in winter from the annual mean concentration of DON, while DOC fluctuated between 23 and 28% at the dairy and the sheep and beef monitoring sites. Similar fluctuations in the concentrations of DON were also observed in the field incubation studies. These results indicate that DON is a dynamic pool of N in soils. There was a strong and significant positive correlation between DON and DOC in pastoral soils (r = 0.71, P < 0.01). There were also significant positive correlations between DON and total soil C (r = 0.59, P < 0.01), total soil N (r = 0.62, P < 0.01) and mineralizable N (r = 0.47, P < 0.01). The rather poor correlations between total soil C and N with DOC and DON, suggest other biogeochemical processes may be influencing concentrations of DOC and DON in these soils. Given the size of DON and DOC pools in the pastoral soils, we suggest that these pools of C and N should be taken into account when assessing the impact of pastoral land use on soil C and N enrichment of surface and groundwater.     

Contribution of α-amino N to extractable organic nitrogen (DON) in three soil types from the Scottish uplands

Organic nitrogen (DON) was extracted from two improved pasture soils, one of which had been re-colonized by acid heath vegetation, and a blanket peat. Although the quantities extracted in H₂O, 10 mM CaCl_2, 500 mM K₂SO₄ and 50 mM Na₂HPO₄ were not consistent, mean extractable DON as a proportion of total N was greater in the two grazed pastures (0.4%) than in the peat (0.2%). Averaged over the four extractants, free α-amino N was greater in the peat and least in the improved pasture soil and accounted for 26% of DON in the peat and less than 5% in the mineral soil. Amino N increased after 6 M HCl hydrolysis, and this combined N contributed 56% to DON in extracts of the mineral soil compared with only 36% in the peat This variation in the relative contributions of free and combined amino N to DON indicated qualitative differences in the composition of DON between the three soils.     

The initial lignin:nitrogen ratio of litter from above and below ground sources strongly and negatively influenced decay rates of slowly decomposing litter carbon pools

Understanding the interactions between the initial biochemical composition and subsequent decomposition of plant litter will improve our understanding of its influence on microbial substrate use to explain the flow of organic matter between soil carbon pools. We determined the effects of land use (cultivation/native woodland/native pasture), litter type (above and below ground) and their interaction on the initial biochemical composition (carbon, nitrogen, water soluble carbon, lignin, tannin and cellulose) and decomposition of litter. Litter decomposition was studied as the mineralization of C from litter by microbial respiration and was measured as CO₂–C production during 105 d of laboratory incubation with soil. A two-pool model was used to quantify C mineralization kinetics. For all litter types, the active C pool decay rate constants ranged from 0.072 d⁻¹ to 0.805 d⁻¹ which represented relatively short half-lives of between 1 and 10 days, implying that this pool contained compounds that were rapidly mineralized by microbes during the initial stages of incubation. Conversely, the decay rate constants for the slow C pool varied widely between litter types within and among land uses ranging from 0.002 d⁻¹ and 0.019 d⁻¹ representing half-lives of between 37 and 446 days. In all litter types, the initial lignin:N ratio strongly and negatively influenced the decay rate of the slow C pool which implied that the interaction between these two litter quality variables had important controls over the decomposition of the litter slow C pool. We interpret our results to suggest that where the flow of C from the active pool to the slow pool is largely driven by microbial activity in soil, the rate of transfer of C will be largely controlled by the quality of litter under different land-use systems and particularly the initial lignin:N ratio of the litter. Compared with native pastures and cultivation, above and below ground litter from native woodland was characterized by higher lignin:N ratio and more slowly decomposing slow C pools which implies that litter is likely to persist in soils, however based on the sandy nature of the soils in this study, it is likely to lack protection from microbial degradation in the long term.     

Recovery of Soil C and N in a Tropical Pasture: Passive and Active Restoration

Soil degradation by deforestation and introduction of exotic grasses is a grave consequence of land‐use change in tropical regions during the last decades. Soil restoration following natural succession (i.e., passive restoration) is slow because of low tree establishment. Introduction of tree plantings by human intervention (i.e., active restoration) results in a promising strategy to accelerate forest succession and soil recovery in tropical region. The present research was carried out to explore the restoration of soil properties after cattle exclusion and of grazing combined with native tree planting introduction (legumes and nonlegumes) in a tropical pasture in Veracruz, southeast Mexico. Results indicate that land‐use changes decreased soil C and N pools in both litter and mineral soil. In addition, soil heterogeneity increased by land‐use changes at both temporal and spatial scales. In the short term, passive succession (i.e., cattle exclusion) favors the recovery of C and N content in labile soil pools, indicated by the increase in litter C and N masses as well as C and N concentrations in soil microbial biomass. Soils under active restoration showed trends to recover the N cycling, such as a greater accumulation of N in litter, in soil total N concentrations, soil microbial biomass N concentrations, rates of net N transformations, and extractable water and microbial biomass C:N ratios mainly under legumes species. Active restoration including legume introduction is a key factor for rapid recovery of soil fertility. Copyright © 2013 John Wiley & Sons, Ltd.     

亚高山林草交错带不同土地利用方式下的土壤氮库和氮转化速率

Soil nitrogen pools (NP), denitrification (DN), gross nitrification (GN), N2O and CO2 flux rates with their responses to temperature increases were determined under five different land uses and managements in a subalpine forest-grassland ecotone of the eastern Tibetan Plateau. Land uses consisted of 1) sparse woodland, 2) shrub-land, 3) natural pasture, 4) fenced pasture, and 5) tilled pasture mimicking a gradient degenerating ecosystem under grazing impacts. The NO3--N content was higher than the NH4+-N content. Comparing tilled pasture with fenced pasture showed that higher intensive management (tillage) led to a significant decrease of soil organic matter (SOM) (P < 0.05) in the soils, which was in contrast to the significant increases (P < 0.05) of DN, GN, N2O and CO2 flux rates. GN (excluding tilled pasture) and CO2 flux rates increased with a temperature rise, but DN and N2O flux rates normally reached their maximum values at 12--14 oC with tilled pasture (the highest management intensity) being very sensitive to temperature increases. There was a difference between net nitrification and GN, with GN being a better indicator of soil nitrification.     

Grassland plants affect dissolved organic carbon and nitrogen dynamics in soil

Dissolved organic carbon (DOC) and nitrogen (DON) are central in many nutrient cycles within soil and they play an important role in many pedogenic processes. Plants provide a primary input of DOC and DON into soil via root turnover and exudation. Under controlled conditions we investigated the influence of 11 grass species alongside an unplanted control on the amount and nature of DOC and DON in soil. Our results showed that while the presence of plants significantly increases the size of a number of dissolved nutrient pools in comparison to the unplanted soil (e.g. DOC, total phenolics in solution) it has little affect on other pools (e.g. free amino acids). Grass species, however, had little effect on the composition of the DOC, DON or inorganic N pools. While the concentration of free amino acids was the same in the planted and unplanted soil, the flux through this pool was significantly faster in the presence of plants. The presence of plants also affected the biodegradability of the DOC pool. We conclude that while the presence of plants significantly affects the quantity and cycling of DOC and DON in soil, comparatively, individual grass species exerts less influence.     

Controls on soil nitrogen cycling and microbial community composition across land use and incubation temperature

We conducted a laboratory incubation of forest (Scots pine (Pinus sylvestris) or beech (Fagus sylvatica)), grassland (Trifolium repens/Lolium perenne) and arable (organic and conventional) soils at 5 and 25 °C. We aimed to clarify the mechanisms of short-term (2-weeks) nitrogen (N) cycling processes and microbial community composition in relation to dissolved organic carbon (DOC) and N (DON) availability and selected soil properties. N cycling was measured by ¹⁵N pool dilution and microbial community composition by denaturing gradient gel electrophoresis (DGGE), phospholipid fatty acid (PLFA) and community level physiological profiles (CLPP). Soil DOC increased in the order of arable<grassland<forest soil while DON and gross N fluxes increased in the order of forest<arable<grassland soil; land use had no affect on respiration rate. Soil DOC was lower, while respiration, DON and gross N fluxes were higher at 25 than 5 °C. Gross N fluxes, respiration and bacterial biomass were all positively correlated with each other. Gross N fluxes were positively correlated with pH and DON, and negatively correlated with organic matter, fungal biomass, DOC and DOC/DON ratio. Respiration rate was positively correlated with bacterial biomass, DON and DOC/DON ratio. Multiple linear modelling indicated that soil pH, organic matter, bacterial biomass, DON and DOC/DON ratio were important in predicting gross N mineralization. Incubation temperature, pH and total-C were important in predicting gross nitrification, while gross N mineralization, gross nitrification and pH were important in predicting gross N immobilization. Permutation multivariate analysis of variance indicated that DGGE, CLPP and PLFA profiles were all significantly (P<0.05) affected by land use and incubation temperature. Multivariate regressions indicated that incubation temperature, pH and organic matter content were important in predicting DGGE, CLPP and PLFA profiles. PLFA and CLPP were also related to DON, DOC, ammonium and nitrate contents. Canonical correlation analysis showed that PLFA and CLPP were related to differences in the rates of gross N mineralization, gross nitrification and soil respiration. Our study indicates that vegetation type and/or management practices which control soil pH and mediate dissolved organic matter availability were important predictors of gross N fluxes and microbial composition in this short-term experiment.     

Land‐Use Effects on Soil Nutrient Cycling and Microbial Community Dynamics in the Everglades Agricultural Area,Florida

Soil subsidence has become a critical problem since the onset of drainage of the organic soils in the Everglades Agricultural Area (EAA), which may impair current land uses in the future. The objectives of this study were to characterize soil microbial community‐level physiology profiles, extracellular enzymatic activities, microbial biomass, and nutrient pools for four land uses: sugarcane, turfgrass, pasture, and forest. Long‐term cultivation and management significantly altered the distribution and cycling of nutrients and microbial community composition and activity in the EAA, especially for sugarcane and turf fields. The least‐managed fields under pasture had the lowest microbial biomass and phosphorus (P) levels. Turf and forest had more microbial metabolic diversity than pasture or the most intensively managed sugarcane fields. Land‐use changes from sugarcane cropping to turf increased microbial activity and organic‐matter decomposition rates, indicating that changes from agricultural to urban land uses may further contribute to soil subsidence.     

Dissolved organic nitrogen dynamics in a Mediterranean vineyard soil

Dissolved organic carbon (DOC) and nitrogen (DON) have been hypothesized to play a central role in nutrient cycling in agricultural soils. The aim of this study was to investigate the annual dynamics of DOC and DON in a Greek vineyard soil and to assess the potential role of DON in supplying N to the vines. Our results indicated that significant quantities of DOC and DON existed in soil throughout the year and that peaks in concentration appeared to correlate with discrete agronomic events (e.g. onset of irrigation and plowing). Both field and laboratory experiments showed that free amino acids were rapidly mineralized in soil and that consequently free amino acids represented only a small proportion of the soil's total soluble N. Due to rapid nitrification the soil solution N was dominated by NO₃⁻. Based upon the calculation of a plant-soil N budget and previous studies on N uptake in Vitis vinifera L., it is likely that DON uptake does not directly supply significant amounts of N to the plant. As the soil was not N limited we hypothesize that amino acids are used by the microbial community more as a source of C rather than a source of N. While we conclude that DON constitutes a significant N pool in vineyard soils further work is required to chemically characterize its constituent units and their relative bioavailability so that their overall role in N cycling can be determined.     

Role of dissolved organic nitrogen (DON) in soil N cycling in grassland soils

Dissolved organic nitrogen (DON) represents a significant pool of soluble N in many soils and freshwaters. Further, the low molecular weight (LMW) component of DON represents an important source of N for microorganisms and can also be utilized directly by some plants. Our purpose was to determine which of the pathways in the decomposition and subsequent ammonification and nitrification of organic N represented a significant block in soil N supply in three agricultural grassland soils. The results indicate that the conversion of insoluble organic N to LMW-DON and not LMW-DON to NH₄⁺ or NH₄⁺ to NO₃⁻ represents a major constraint to N supply. We hypothesize that there are two distinct DON pools in soil. The first pool comprises mainly free amino acids and proteins and is turned over very rapidly by the microbial community, so it does not accumulate in soil. The second pool is a high molecular weight pool rich in humic substances, which turns over slowly and represents the major DON loss to freshwaters. The results also suggest that in NO₃⁻ rich soils the uptake of LMW-DON by soil microorganisms may primarily provide them with C to fuel respiration, rather than to satisfy their internal N demand.     

Soil carbon and related soil properties along a soil type and land‐use intensity gradient,New South Wales,Australia

The aim of this study was to systematically quantify differences in soil carbon and key related soil properties along a replicated land‐use intensity gradient on three soil landscapes in northwest New South Wales, Australia. Our results demonstrate consistent land‐use effects across all soil types where C, N and C:N ratio were in the order woodland > unimproved pasture = improved pasture > cultivation while bulk density broadly showed the reverse pattern. These land‐use effects were largely restricted to the near surface soil layers. Improved pasture was associated with a significant soil acidification, indicating that strategies to increase soil carbon through pasture improvement in these environments might also have associated soil degradation issues. Total soil carbon stocks were significantly larger in woodland soils, across all soil types, compared with the other land‐uses studied. Non‐wooded systems, however, had statistically similar carbon stocks and this pattern persisted whether or not carbon quantity was corrected for equivalent mass. Our results suggest that conversion from cultivation to pasture in this environment would yield between 0.06 and 0.15 t C/ha/yr which is at the lower end of predicted ranges in Australia and well below values measured in other cooler, wetter environments. We estimate that a 10% conversion rate (cultivation to pasture) across NSW would yield around 0.36 Mt CO₂‐e/yr which would contribute little to emission reductions in NSW. We conclude that carbon accumulation in agricultural soils in this environment might be more modest than current predictions suggest and that systematically collected, regionally specific data are required for the vegetation communities and full range of land‐uses before accurate and reliable predictions of soil carbon change can be made across these extensive landscapes.     

Microbial decomposition of leached or extracted dissolved organic carbon and nitrogen from pasture soils

Microbial decomposition of extracted and leached dissolved organic carbon (DOC) and nitrogen (DON) was demonstrated from three pasture soils in laboratory incubation studies. DOC concentration in water extracts ranged between 29 and 148 mg C L^?1 and DON concentration ranged between 2 and 63 mg N L^?1. Between 17 and 61 % of the DOC in the water extracts were respired as CO₂ by microbes by day 36. DON concentrations in the extracts declined more rapidly than DOC. Within the first 21 days of incubation, the concentration of DON was near zero without any significant change in the concentration of NO₃ ^? or NH₄ ⁺, indicating that microbes had utilized the organic pool of N preferentially. Decomposition of leached DOC (ranged between 7 and 66 mg C L^?1) and DON (ranged between 6 and 11 mg N L^?1) collected from large lysimeters (with perennial pasture; 50 cm diameter?×?80 cm deep) followed a similar pattern to that observed with soil extracts. Approximately 28 to 61 % of the DOC in leachates were respired as CO₂ by day 49. The concentration of DON in the leachates declined to below 1 mg N L^?1 within 7–14 days of the incubation, consistent with the observations made with extractable DON. Our results clearly show that DOC and DON components of the dissolved organic matter in pasture soils, whether extracted or leached, are highly decomposable and bioavailable and will influence local ecosystem functions and nutrient balances in grazed pasture systems and receiving water bodies.     

Characterization of Soil Phosphorus in Differently Managed Clay Soil by Chemical Extraction Methods and 31P NMR Spectroscopy

Changes in land use alter the natural cycling of phosphorus (P) in soil. Understanding the chemical nature of these changes is important when developing sustainable management practices for cultivated soils. In this study, we evaluated the ability of commonly used laboratory methods to characterize land use–induced changes in various P pools. Also, the characteristics of soil P revealed by different methods are discussed. Soil samples were taken from three differently managed field plots of the same clay soil: uncultivated grassland and organic and conventional crop rotations. Soil P reserves were characterized using Chang and Jackson and Hedley sequential fractionation procedures and by sodium hydroxide (NaOH)–ethylenediaminetetraacetic acid (EDTA) extraction followed by ³¹P NMR spectroscopy. Both of the tested fractionation methods identified differences in the P pools and provided evidence regarding land use–induced changes. However, the ³¹P NMR analysis suggests that the quality of organic P in this soil was not affected by the change in land use.     

Dissolved organic matter leaching in some contrasting New Zealand pasture soils

Leaching of dissolved organic matter (DOM) from pastoral soils is increasingly seen as an important but poorly understood process. This paper examined the relationship between soil chemical properties, microbial activity and the losses of dissolved organic carbon (DOC) and nitrogen (DON) through leaching from six pasture soils. These soils differed in carbon (C) (4.6–14.9%) and nitrogen (N) (0.4–1.4%) contents and in the amount of organic C and N that had accumulated or been lost in the preceding 20+ years (i.e. −5131 to +1624 kg C ha⁻¹ year⁻¹ and −263 to +220 kg N ha⁻¹ year⁻¹, respectively). The paper also examined whether between‐soil‐type differences in DOC and DON leaching was a major explanatory factor in the observed range of soil organic matter (SOM) changes in these soils. Between 280 and 1690 kg C ha⁻¹ year⁻¹ and 28–117 kg N ha⁻¹ year⁻¹ leached as DOC and DON, respectively, from the six soils in a lysimeter study, with losses being greater from two poorly drained gley soils. Losses of C and N of this magnitude, while at the upper end relative to published data, could not fully explain the losses at Rawerawe, Bruntwood and Lepperton sites reported by Schipper et al. (2007) . The study highlights the leaching of DOM as a significant pathway of loss of C and N in pasture soils that is often ignored or given little attention in predictive models and nutrient budgeting. Leaching losses of DOC and DON alone, or in combination with slightly increased respiration losses of SOM given a 0.2°C increase in the mean annual soil temperature, do not fully explain long‐term changes in the SOM observed at these sites. When soils examined in the present study were separated on the basis of drainage class, the losses of DOC by leaching were correlated with both total and hot‐water extractable C (HWC), the latter being a measure of the labile SOM fraction. Basal microbial CO₂ respiration rates, which varied between 1 and 3.5 µg CO₂‐C g⁻¹ soil hour⁻¹ in surface soils (0–75‐mm depth), was also linked to HWC and the quantities of C lost as DOC. Adoption of the HWC method as an approach that could be used as a proxy for the direct measurement of the soil organic C lost by leaching as DOC or respired needs to be examined further with a greater number of soils. In comparison, a poor relationship was found between the hot‐water extractable N (HWN) and loss of DON by leaching, despite HWN previously being shown to be a measure of the mineralizable pool of N in soils, possibly reflecting the greater competition for N than C in these soils.     

上海郊区园艺土壤氮素的生物形成动态变化

Dissolved organic nitrogen (DON) represents a significant pool of soluble nitrogen (N) in soil ecosystems. Soil samples under three different horticultural management practices were collected from the Xiaxiyang Organic Vegetable and Fruit Farm, Shanghai, China, to investigate the dynamics of N speciation during 2 months of aerobic incubation, to compare the effects of different soils on the mineralization of ¹⁴C-labeled amino acids and peptides, and to determine which of the pathways in the decomposition and subsequent ammonification and nitrification of organic N represented a significant blockage in soil N supply. The dynamics of N speciation was found to be significantly affected by mineralization and immobilization. DON, total free amino acids, and NH₄⁺-N were maintained at very low levels and did not accumulate, whereas NO₃^--N gradually accumulated in these soils. The conversion of insoluble organic N to low-molecular-weight (LMW) DON represented a main constraint to N supply, while conversions of LMW DON to NH₄⁺-N and NH₄⁺-N to NO₃^--N did not. Free amino acids and peptides were rapidly mineralized in the soils by the microbial community and consequently did not accumulate in soil. Turnover rates of the additional amino acids and peptides were soil-dependent and generally followed the order of organic soil > transitional soil > conventional soil. The turnover of high-molecular-weight DON was very slow and represented the major DON loss. Further studies are needed to investigate the pathways and bottlenecks of organic N degradation.     

Carbon and nitrogen mineralization in cultivated and natural savanna soils of Northern Tanzania

In the present study, soil C and N mineralization and nutrient availability were compared: (1) in savanna woodland soils under natural acacia vegetation; (2) at termite sites; (3) in degraded woodland where acacias were selective logged for charcoal production; (4) in agricultural fields which were cultivated for 3 and 15 years, and (5) in traditional homestead fields which regularly received animal manure for about 10 years. Soil C and N mineralization dynamics were measured by incubation under controlled conditions for 120 days. Labile and stable soil C and N pools were determined by fitting double-exponential models to the measured cumulative mineralization. Selective removal of acacias from the woodland and short-term cultivation for 3 years did not affect available nutrient contents but significantly decreased total C and N contents and mineralization (P<0.05). Mainly the labile soil N pool decreased during the first 3 years of continuous cropping, whereas after 15 years the stable N pool, total S, available Ca and Zn contents were also depleted. Even after 15 years, however, the decrease of nutrient availability (apart from N) was less severe than that of soil organic matter stability. Additionally, not only the labile but also the more stable soil C and N pools decreased and controlled total mineralization as determined by the incubation experiments. Homestead fields with manure additions were shown to have elevated soil nutrient and organic matter contents. However, the manure should be mixed into the soil to improve organic matter stabilization. Soil regeneration in degraded savannas and recently cultivated fields might rapidly be achieved, whereas the 15-years-cultivated fields may require longer fallows to restore soil fertility.     

Changes in soil N status and N supply rates in agricultural land afforested with eucalypts in south-western Australia

In south-western Australia, plantations of Eucalyptus globulus are being established on land that has previously been used for conventional agriculture. Sustaining the productivity of these tree plantations in second and subsequent rotations will depend partly on maintenance of soil fertility, especially soil nitrogen (N) supply rates. We compared soil N status and supply rates between adjacent pasture and 6-11 year old first-rotation eucalypt plantations at 31 paired sites in south-western Australia. Total soil N varied widely among sites (0.07-0.68% in the fraction <2 mm of the 0-10 cm soil layer), but concentrations averaged over all sites did not differ between land-use types. However, measurements of the indices of mineralization (mineral N produced during incubation of intact cores), potentially available N (from short-term anaerobic incubation) and model-predicted mineralization rates during 28-day aerobic incubations were generally lower in afforested soils than in pasture soils. This finding was supported by in situ field estimates of N mineralization over a full year at two contrasting paired pasture-plantation sites. At each site there was a marked reduction (2-3-fold) in net annual mineral N flux rates in soils under eucalypt plantations. Reduced N mineralization associated with tree plantations was due to both changes in soil organic matter quality and the generally lower soil moisture content under trees in comparison with pasture. These results suggest that N supply rates of pasture soils are likely to decline when the land is planted to successive crops of eucalypts. Eucalypt plantation managers will need to take account of this and implement management strategies to maintain adequate N nutrition to sustain tree growth in future rotations.     

Dissolved organic carbon and nitrogen dynamics in temperate coniferous forest plantations

Dissolved organic matter (DOM) plays a central role in driving many chemical and biological processes in soil; however, our understanding of the fluxes and composition of the DOM pool still remains unclear. In this study we investigated the composition and dynamics of dissolved organic carbon (DOC) and nitrogen (DON) in five temperate coniferous forests. We subsequently related our findings to the inputs (litterfall, throughfall, atmospheric deposition) and outputs (leaching, respiration) of C and N from the forest and to plant available sources of N. With the exception of NO₃^?, most of the measured soil solution components (e.g. DOC, DON, NH₄⁺, free amino acids, total phenolics and proteins) progressively declined in concentration with soil depth, particularly in the organic horizons. This decline correlated well with total microbial activity within the soil profile. We calculated that the amount of C lost by soil respiration each day was equivalent to 70% of the DOC pool and 0.06% of the total soil C. The rapid rate of amino acid mineralization and the domination of the low molecular weight soluble N pool by inorganic N suggest that the microbial community is C‐ rather than N‐limited and that C‐limitation increases with soil depth. Further, our results suggest that the forest stands were not N‐limited and were probably more reliant on inorganic N as a primary N source rather than DON. In conclusion, our results show that the size of the DON and DOC pools are small relative to both the amount of C and N passing through the soil each year and the total C and N present in the soil. In addition, high rates of atmospheric N deposition in these forests may have removed competition for N resources between the plant and microbial communities.     

Tillage effects on microbial biomass and nutrient distribution in soils under rain-fed corn production in central-western Mexico

Quantifying how tillage systems affect soil microbial biomass and nutrient cycling by manipulating crop residue placement is important for understanding how production systems can be managed to sustain long-term soil productivity. Our objective was to characterize soil microbial biomass, potential N mineralization and nutrient distribution in soils (Vertisols, Andisols, and Alfisols) under rain-fed corn (Zea mays L.) production from four mid-term (6 years) tillage experiments located in central-western, Mexico. Treatments were three tillage systems: conventional tillage (CT), minimum tillage (MT) and no tillage (NT). Soil was collected at four locations (Casas Blancas, Morelia, Apatzingán and Tepatitlán) before corn planting, at depths of 0–50, 50–100 and 100–150 mm. Conservation tillage treatments (MT and NT) significantly increased crop residue accumulation on the soil surface. Soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were highest in the surface layer of NT and decreased with depth. Soil organic C, microbial biomass C and N, total N and extractable P of plowed soil were generally more evenly distributed throughout the 0–150 mm depth. Potential N mineralization was closely associated with organic C and microbial biomass. Higher levels of soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were directly related to surface accumulation of crop residues promoted by conservation tillage management. Quality and productivity of soils could be maintained or improved with the use of conservation tillage.     

Land-Use Effect on Selected Soil Quality Parameters

Land use is a key factor that affects soil quality. The purpose of the present study was to investigate changes of selected soil chemical properties related to soil function under different land uses. Five experimental sites arranged in a complete randomized blocks located within the Kalloni watershed (Lesvos Island, Greece) corresponding to different land uses (Pinus brutia forest, brushwood pasture, rain-fed olive grove, wheat, and maize) were compared for soil pH, electrical conductivity (EC), total nitrogen (N), nitrate N (NO₃-N), Olsen phosphorus (P), and organic matter. Soil nitrate and P concentration were in the order corn > wheat > olive > pasture > forest. Soil EC and NO₃-N showed significant within-year variability only in the corn and wheat systems. Corn and wheat had the lowest soil organic-matter content followed the order forest > pasture > olive grove = wheat = corn. However, total N did not significantly differ among land uses.