A Rapid Chemical Method for Estimating Potentially Mineralizable and Particulate Organic Nitrogen in Mollisols

The objective of this study was to obtain an indicator of labile nitrogen (N) through a cost- and time-saving procedure by evaluating the relationships among potentially mineralizable N (N₀), particulate organic matter N (POM-N) and soil organic N extracted through partial soil digestion with different concentrations of sulfuric acid (H₂SO₄). Soil sampling (0–20 cm) was from nine fields under no-tillage. The N₀ and POM-N were determined by long-term aerobic incubation and soil physical fractionation, respectively. A simple chemical method was developed by soil digestion at 100°C for 4 h with different concentrations of H₂SO₄ (0.1, 0.5, 1, 6, 12, and 24 mol L^?1). All acid concentrations showed significant relationships with N₀ as POM-N; however, the best prediction was resulted for 0.5 mol L^?1 (R² = 0.90–0.94, respectively), thus using this methodology as soil labile N indicator. This method would optimize N₀ and POM-N estimation in short term and at a low cost.     

Soil Organic Carbon and Nitrogen Fractions under Different Land Uses and Tillage Practices

Many questions have surfaced regarding long-term impacts of land-use and cultivation system on soil carbon (C) sequestration. The experiment was conducted at Ohio Agricultural Research and Development Center. Only minor variations of soil organic carbon (SOC) and nitrogen (N) fractions with depth under plow tillage (PT). The SOC, total nitrogen (TN), microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) concentrations were higher under grassland and forestland in the top 0–15 cm depth than arable soils. No-tillage (NT) also increased SOC and N fractions concentrations in the surface soils than PT. Compared to arable, grass and forest could significantly improve proportions of MBC and MBN, and reduce proportions of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). NT and forest also increased the ratio of SOC/TN, MBC/MBN, and DOC/DON. Overall, grass and forest provided more labile C and improved C sequestration than arable. So did NT under arable land-use.     

A rapid method for estimating labile carbon and nitrogen pools in Mollisols under no-tillage

The objective of this study was to adapt the partial chemical digestion method for estimation of labile soil organic matter pools by evaluating the effect of different digestion times in Mollisols of the Argentine Pampas. The soils were sampled from nine agricultural fields under no-tillage at the 0–20 cm depth. A chemical method was performed through partial soil digestion with dilute sulphuric acid at 100°C on the basis of four digestion times: 1 (N_d1), 2 (N_d2), 4 (N_d4) and 6 (N_d6) hours. Soil organic carbon (C) and nitrogen (N) fractions were determined. The extracted organic N (N_d) ranged from 0.076 g kg^?1 to 0.273 g kg^?1, with a mean of 0.154 g kg^?1. Statistically, the means for each digestion time indicated highly significant differences (P = 0.008). High correlations were found between N_d for different times and labile C and N fractions. However, the best fit prediction was observed between N_d2 and soil total carbohydrates (CHt), with a high coefficient of determination (R² = 0.94). Partial chemical digestion for 2 h can be used as a rapid indicator to accurately predict CHt. Because of its speed and simplicity, this method may also be useful for rapid soil quality assessments.     

Changes in amount of organic and inorganic fractions of nitrogen in an Eutrochrept soil after long‐term cropping with different fertilizer and organic manure inputs

The influence of 30 years of cropping with different fertilizer and farmyard manure (FYM) inputs on the contents and depth distribution of organic C, total N (N_t), soil mineralizable N, and organic and inorganic N fractions was investigated in an Eutrochrept. Continuous application of 100 %NPK(+S), 150 %NPK(+S), and 100 %NPK(+S)+FYM led to a marked increase in organic C, total N, hydrolyzable N (viz., amino acid N, hydrolyzable NH₄‐N, hexose amine N, and unidentified hydrolyzable N), and nonhydrolyzable N as compared to an adjacent fallow. The contents of the various organic N fractions were largest in surface soil and thereafter decreased with the depth. However, at 30 – 45 cm depth the content of organic C was not affected by the different treatments except 100 %NPK(+S)+FYM. On the other hand, continuous cropping without fertilization resulted in a depletion of total hydrolyzable N in control over fallow by 27.2 % (0–15 cm), 19.6 % (15–30 cm), and 4.7 % (30–45 cm). The incorporation of FYM with 100 %NPK(+S) resulted in greater contents of soil mineralizable N as compared to 100 %NPK(+S) (0–15, 15–30 cm). The proportion of hydrolyzable N (57–76 % of N_t) decreased and that of nonhydrolyzable N (22–40 % of N_t) increased with depth. The proportion of amino acid N (19–26 % of N_t), hexose amine N (2.1–3.5 % of N_t) and unidentified hydrolyzable N (17–27 % of N_t) decreased with depth. All organic soil N fractions including even nonhydrolyzable N in surface and subsurface soils were highly significantly correlated with soil mineralizable N derived from incubations under waterlogged and aerobic conditions. The best correlation to mineralizable N was found for amino acid N and the least significant correlation for nonhydrolyzable N.     

Long-Term Tillage,Straw Management,and Nitrogen Fertilization Effects on Organic Matter and Mineralizable Carbon and Nitrogen in a Black Chernozem Soil

Soil, crop, and fertilizer management practices may affect quality of organic carbon (C) and nitrogen (N) in soil. A long-term field experiment (growing barley, wheat, or canola)was conducted on a Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, to determine the influence of 19 years (1980 to 1998) of tillage [zero tillage (ZT) and conventional tillage (CT)], straw management [straw removed (S_Rem) and straw retained (S_Ret)], and N fertilizer rate (0, 50, and 100 kg N ha^?1 in S_Ret and 0 kg N ha^?1 in S_Rem plots) on macro-organic matter C (MOM-C) and N (MOM-N), microbial biomass C (MB-C), and mineralizable C (C_min) and N (N_min) in the 0- to 7.5-cm and 7.5- to 15-cm soil layers. Treatments with N fertilizer and S_Ret generally had a greater mass of MOM-C (by 201 kg C ha^?1 with 100 kg N ha^?1 rate and by 254 kg C ha^?1 with S_Ret), MOM-N (by 12.4 kg N ha^?1 with 100 kg N ha^?1 rate and by 8.0 kg N ha^?1 with S_Ret), C_min(by 146 kg C ha^?1 with 100 kg N ha^?1 rate and by 44 kg C ha^?1 with S_Ret), and N_min(by 7.9 kg N ha^?1 with 100 kg N ha^?1 rate and by 9.0 kg N ha^?1 with S_Ret)in soil than the corresponding zero-N and S_Rem treatments. Tillage, straw, and N fertilizer had no consistent effect on MB-C in soil. Correlations between these dynamic soil organic C or N fractions were strong and significant in most cases, except for MB-C, which had no significant correlation with MOM-C and MOM-N. Linear regressions between crop residue C input and mass of MOM-C, MOM-N, C_min, and N_min in soil were significant, but it was not significant for MB-C. The effects of management practices on dynamic soil organic C and N fractions were more pronounced in the 0- to 7.5-cm surface soil layer than in the 7.5- to 15-cm subsoil layer. In conclusion, the findings suggest that application of N fertilizer and retention of straw would improve soil quality by increasing macro-organic matter and N-supplying power of soil.     

Hot water extractable C and N in relation to microbiological properties of soils under beech forests

Hot water extraction is sometimes recommended as an easy method to estimate the readily mineralizable fractions of total C (C_t) and total N (N_t) in arable soils. However, the usefulness of this method for forest soils has not been adequately studied. The objectives of this study were to relate the hot water extractable C (C_hw) and N (N_hw) to microbiological and chemical properties of the forest soils under beech (Fagus sylvatica L.) stands and to test the ability of near infrared spectroscopy (NIRS) to predict chemical and microbial properties of these soils. Soils differing in humus type, soil type and soil texture were collected from five locations and five depths. In all soils the amount of C_hw was higher than the microbial biomass C (C_mic) indicating that a considerable part of C_hw was of non-microbial origin. The amount of C_hw in mineral soil correlated significantly (r =–0.30–0.53) with C_mic, basal respiration (BAS) and C_t/N_t ratio but was not related to C_mic/C_t ratio. The amount of N_hw was correlated with C_mic, BAS, C_mic/C_t ratio, and C_t/N_t ratio (r =–0.59–0.78). However, C_t and N_t values showed better relationships (r =–0.42–0.88) with all the parameters, indicating no advantage in using C_hw and N_hw in forest soils. NIRS predicted satisfactorily C_t, N_t, C_hw, N_hw, C_mic, C_mic/C_t ratio and BAS in the mineral soils [the regression coefficients (a) of linear regression (measured against predicted values) ranged from 0.84 to 1.17 and the correlation coefficients (r) ranged from 0.86 to 0.94] indicating the applicability of NIRS to estimate these properties.     

珠江三角洲平原农田土壤有机碳组分及剖面分布特征

采集珠江三角洲平原20个农田不同发生层的土壤样品,研究了土壤有机碳(SOC)及其组分,包括高锰酸钾氧化碳(POXC)、不同浓度硫酸氧化的有机碳组分(CF1、CF2、CF3和CF4)、胡敏酸碳(HAC)、富里酸碳(FAC)和胡敏素碳(HMC)的分布特征及其相互关系和影响因素。结果表明:SOC含量变幅为1.88~52.78 g kg~(-1)。SOC、CF1、CF2、CF3、POXC、FAC、HAC含量均在耕作层(Ap1)中最高,平均值分别为17.87、7.29、4.32、2.57、0.58、1.72、2.60 g kg~(-1),在水耕氧化还原层(Br)中最低;CF4和HMC含量在Bg层土壤中最高,平均值分别为5.32、15.03 g kg~(-1)。Ap1、Ap2、Br、Bg层中POXC占有机碳比例平均值分别为3.2%、3.3%、3.5%、3.6%,活性有机碳(CF1+CF2)是土壤有机碳的主要组分,在各发生层土壤中所占比例均60%。土壤腐殖质以HMC为主,所占比例约80%。土壤有机碳及其各组分之间均呈极显著正相关,土壤有机碳及其组分与土壤全氮、碱解氮呈极显著正相关,与pH值呈显著负相关。     

Can Near or Mid‐Infrared Diffuse Reflectance Spectroscopy Be Used to Determine Soil Carbon Pools?

Abstract

The objective of this study was to compare mid‐infrared (MIR) an near‐infrared (NIR) spectroscopy (MIRS and NIRS, respectively) not only to measure soil carbon content, but also to measure key soil organic C (SOC) fractions and the δ¹³C in a highly diverse set of soils while also assessing the feasibility of establishing regional diffuse reflectance calibrations for these fractions. Two hundred and thirty‐seven soil samples were collected from 14 sites in 10 western states (CO, IA, MN, MO, MT, ND, NE, NM, OK, TX). Two subsets of these were examined for a variety of C measures by conventional assays and NIRS and MIRS. Biomass C and N, soil inorganic C (SIC), SOC, total C, identifiable plant material (IPM) (20× magnifying glass), the ratio of SOC to the silt+clay content, and total N were available for 185 samples. Mineral‐associated C fraction, δ¹³C of the mineral associated C, δ¹³C of SOC, percentage C in the mineral‐associated C fraction, particulate organic matter, and percentage C in the particulate organic matter were available for 114 samples. NIR spectra (64 co‐added scans) from 400 to 2498 nm (10‐nm resolution with data collected every 2 nm) were obtained using a rotating sample cup and an NIRSystems model 6500 scanning monochromator. MIR diffuse reflectance spectra from 4000 to 400 cm^?1 (2500 to 25,000 nm) were obtained on non‐KBr diluted samples using a custom‐made sample transport and a Digilab FTS‐60 Fourier transform spectrometer (4‐cm^?1 resolution with 64 co‐added scans). Partial least squares regression was used with a one‐out cross validation to develop calibrations for the various analytes using NIR and MIR spectra. Results demonstrated that accurate calibrations for a wide variety of soil C measures, including measures of δ¹³C, are feasible using MIR spectra. Similar efforts using NIR spectra indicated that although NIR spectrometers may be capable of scanning larger amounts of samples, the results are generally not as good as achieved using MIR spectra.     

Organic carbon stocks in topsoil and subsoil in long-term ley and cereal monoculture rotations

Soil organic C (SOC) in agricultural soils is affected by cropping system. In order to compare the impact of ley-dominated and cereal monoculture rotations on soil properties, a series of experiments was started in Sweden in the early 1980s. This study quantified the effect of rotations and N fertilisation on SOC stocks and microbial community composition. After 35 years, soil samples taken down to 50-cm depth at two sites with contrasting soil texture (clay and loam) were analysed for soil C and N, bulk density and phospholipid fatty acid (PLFA) content. Significant increases in SOC concentrations and stocks were found in the ley-dominated rotation compared with the cereal monoculture rotation, the difference being 0.36 and 0.59 Mg C ha^?1 year^?1 in the topsoil (0–20 cm) for sites with the clay and loam texture, respectively, in average over N fertiliser levels. Nitrogen fertilisation increased SOC stocks significantly in the cereal monoculture, but not in the ley-dominated rotation. In the loam, SOC responses in the subsoil were almost as high as those in the topsoil, but they were insignificant in the clay soil. These results indicate that soil texture and structure can have a great impact on the potential of subsoils to sequester C, which requires attention when scaling up SOC sequestration rates for regional or global assessments.     

Effect of grassland harvesting frequency and N-fertilization on stocks and dynamics of soil organic matter in the temperate climate

ABSTRACT

Management of grassland may affect the dynamics of soil organic carbon (SOC). Objectives were to analyze the effect of different harvesting frequencies and nitrogen fertilization regimes on SOC and total N stocks in a field trial on a sandy loam to loamy sand soil of a grassland site near Kiel (Germany). Additionally, effects on microbial biomass C (C_mic) and ergosterol (as proxy for fungi) contents, water-stable aggregate size-classes and density fractions were studied. In the surface soil (0–10 cm), SOC and total N stocks, amounts of large water-stable macroaggregates (> 2000 µm) and contents of C_mic and ergosterol were significantly higher under a five cut regime. C_mic (r_Spearman = 0.61) and ergosterol contents (r_Spearman = 0.67) were correlated with amounts of large water-stable macroaggregates suggesting that fungi and microbial biomass play an important role in binding of small macroaggregates into large macroaggregates. The free light fraction of SOM showed significantly higher C concentrations under three cut compared to five cut at 30–60 cm, presumably related to the C/N ratio and the decomposability of root litter. This study indicates the importance of cutting frequency on SOC and total N stocks, amounts of large macroaggregates and contents of C_mic and ergosterol.     

Environmental and site factors controlling the vertical distribution and radiocarbon ages of organic carbon in a sandy soil

Soil organic carbon (SOC) content and radiocarbon concentration were measured in three particle-size fractions and charcoal fragments at four depths to bedrock in a sandy soil from SE Australia. SOC content declined with depth for all fractions. The enrichment factors of SOC showed that the finest particles are most important for SOC storage throughout the soil profile, and their importance for SOC storage increased with depth. In the topsoil, all particle-size fractions contained modern SOC. In contrast, charcoal from this depth gave radiocarbon ages of 85–165 years Before Present (BP). This difference was more pronounced at 30–60 cm, where the charcoal was dated at 2,540 years BP, over 12 times as old as the youngest fraction at that depth. These results confirm charcoal as a highly stable form of SOC. The radiocarbon data in the topsoil and near bedrock indicate that neither microaggregation nor mineral association is important for SOC stability in this soil. At intermediate sampling depths, the mid-sized fraction was the oldest. We believe that this is the result of charcoal accumulation in this fraction, inducing a shift in radiocarbon age. However, near bedrock (100–120 cm), radiocarbon concentration did not differ significantly between fractions, despite greater SOC retention in smaller fractions. In addition, radiocarbon ages at 100–120 cm indicate that charcoal is not present at this depth. We propose that environmental and soil conditions (substrate limitation, water and oxygen availability, and temperature) are responsible for the stabilization of SOC at this depth, where SOC concentrations were very low (0.1–0.3 %). Our results demonstrate that, although fine particles retain more SOC than coarse ones, they do not stabilize SOC in this sandy soil. Instead, environmental (bushfires and climate) and site factors (soil texture and soil mineralogy) control the distribution and stability of SOC throughout the soil profile.     

有机肥对棕壤不同粒级有机碳和氮的影响

采集棕壤长期肥料定位试验站不施肥和施用不同用量有机肥的土壤,通过超声波分散—离心分离得到细黏粒(<0.2μm)、粗黏粒(0.2～2μm)、粉粒(2～53μm)、细砂粒(53～250μm)和粗砂粒(250～2000μm)5个颗粒级别后,分析全土及不同粒级中土壤有机碳和氮并进行含量与分布的比较。结果表明,有机质主要分布于黏粒级中,其含量占全土有机碳的42.8%、全氮的58.3%,碳氮比随着粒级的增加而逐渐增大,表明氮易于在小粒级中富集。长期施用有机肥后,全土及各粒级有机碳和氮含量均有显著增加;砂粒级中有机碳和氮的富集系数升高,黏粒级中富集系数降低,粉粒级和砂粒级中的碳氮比降低。增加有机肥的用量加强了全土和各粒级对有机碳和氮的积累,同时加强了粉粒级和砂粒级碳氮比降低的程度。     

Nitrogen Management for Sustainable Soil Organic Carbon Increase in Inceptisols Under Wheat Cultivation

Application of nitrogen (N) fertilizers to increase crop yield is a worldwide practice, which also has a positive influence on the soil organic carbon (SOC) increase. This study was carried out to investigate the dynamics of SOC and its fractions under different levels of N fertilization in wheat grown inceptisols of Northeast India over a period of 2 years. For the purpose of this study, fertilizer treatments with five N levels (40, 60, 72, 80, and 100 kg N ha^?1) were applied in randomized block design. Increased SOC particulate organic carbon (POC), humic acid carbon, and fulvic acid carbon were recorded under application of higher N. Stability of SOC as indicated by E4/E6 ratio and microbial biomass carbon (MBC) was higher on application of 72 kg N ha^?1. Among the SOC fractions, POC and MBC respond rapidly to different N fertilization rates. Available N and phosphatase activity increased while pH and urease activity (UA) decreased as a function of applied N fertilizer levels. Nitrogen fertilization increased wheat yield and biomass with insignificant differences among 100, 80, and 72 kg N ha^?1. Thus, under the present experimental conditions, application of 72 kg N ha^?1 can sustain SOC and soil health without compromising wheat yield in the inceptisols of Northeast India.     

Dryland plant biomass and soil carbon and nitrogen fractions on transient land as influenced by tillage and crop rotation

Soil and crop management practices may alter the quantity, quality, and placement of plant residues that influence soil C and N fractions. We examined the effects of two tillage practices [conventional till (CT) and no-till (NT)] and five crop rotations [continuous spring wheat (Triticum aestivum L.) (CW), spring wheat–fallow (W–F), spring wheat–lentil (Lens culinaris Medic.) (W–L), spring wheat–spring wheat–fallow (W–W–F), and spring wheat–pea (Pisum sativum L.)–fallow (W–P–F)] on transient land previously under 10 years of Conservation Reserve Program (CRP) planting on the amount of plant biomass (stems + leaves) returned to the soil from 1998 to 2003 and soil C and N fractions within the surface 20 cm in March 2004. A continued CRP planting was also included as another treatment for comparing soil C and N fractions. The C and N fractions included soil organic C (SOC), soil total N (STN), microbial biomass C and N (MBC and MBN), potential C and N mineralization (PCM and PNM), and NH₄-N and NO₃-N contents. A field experiment was conducted in a mixture of Scobey clay loam (fine-loamy, mixed, Aridic Argiborolls) and Kevin clay loam (fine, montmorillonitic, Aridic Argiborolls) in Havre, MT, USA. Plant biomass yield varied by crop rotation and year and mean annualized biomass was 45–50% higher in CW and W–F than in W–L. The SOC and PCM were not influenced by treatments. The MBC at 0–5 cm was 26% higher in W–W–F than in W–F. The STN and NO₃-N at 5–20 cm and PNM at 0–5 cm were 17–1206% higher in CT with W–L than in other treatments. Similarly, MBN at 0–5 cm was higher in CT with W–L than in other treatments, except in CT with W–F and W–P–F. Reduction in the length of fallow period increased MBC and MBN but the presence of legumes, such as lentil and pea, in the crop rotation increased soil N fractions. Six years of tillage and crop rotation had minor influence on soil C and N storage between croplands and CRP planting but large differences in active soil C and N fractions.     

Invasion of Praxelis clematidea increases the chemically non-labile rather than labile soil organic carbon in a tropical savanna

Soil organic carbon (SOC) consists of various classes of organic substances that can be pooled as labile and non-labile fractions. Previous studies have suggested that plant invasion increases SOC content, but whether invasion consistently alters SOC fractions remains unclear. Consequently, the present study was conducted to observe the effects of Praxelis clematidea invasion on SOC fractions in a tropical savanna of southern China. Soil samples were collected in two surface soil layers (0–10 and 10–20 cm) from non-, slightly and severely invaded plots to analyse the total SOC, readily oxidizable SOC (ROC), and non-readily oxidizable SOC (NROC) content. The results showed that severe P. clematidea invasion significantly increased the SOC content by 47% in the surface soil (p < 0.001). The increase in SOC content largely originated from the accumulation of NROC (the non-labile fraction), rather than ROC which typically is regarded as the labile OC fraction. This change may be beneficial to long-term soil C stabilization because chemical recalcitrance is an important pathway to prevent SOC from decomposition. Although the mechanisms for NROC accumulation have not been thoroughly elucidated to date, our results suggest that P. clematidea invasion may facilitate soil C sequestration in this tropical savanna.     

Long-Term Effect of Nitrogen and Tillage Management on Soil Carbon Pools in the Semiarid Northern Great Plains

No-tillage and manure application effect on soil organic carbon (SOC) and total nitrogen (N) concentrations were studied under a 27-year-old 4-year rotation consisting corn (Zea mays L.)-soybean (Glycine max L.)-wheat (Triticum aestivum L.)-field pea (Pisum sativum L.). Under each crop, four applied N treatments were control, annual urea-N applications at the rate of 45 and 89 kg N ha^?1, and composted beef cattle feedlot manure-N at the rate 179 kg N ha^?1 applied once every four year. For each fertilizer treatment, no-till (NT) and conventional till (CT) were compared for basic soil properties, SOC, and total N within 0–15 cm soil. Manure application significantly reduced soil bulk density and increased SOC and total N over urea-N. Particulate organic matter, mineralizable N, and permanganate-oxidizable C fractions significantly related with SOC. Long-term manure additions and no-tillage had potential to improve soil compaction and maintain SOC over chemical fertilizer N and CT.     

Use of different chemometric approaches for an estimation of soil properties at field scale with near infrared spectroscopy

Visible and near infrared spectroscopy (vis‐NIRS) may be useful for an estimation of soil properties in arable fields, but the quality of results are often variable depending on the applied chemometric approach. Partial least squares regression (PLSR) may be replaced by approaches which employ supervised learning methods or variable selection procedures in order to increase the proportion of informative wavelengths used in the estimation procedure, to reduce the noise of the spectra and to find the best fitting solution. Objectives were (1) to compare the usefulness of PLSR with either PLSR combined with a genetic algorithm (GA‐PLSR) or support vector machine regression (SVMR) for an estimation of soil organic carbon (SOC), total nitrogen (N), pH, cation exchange capacity (CEC) and soil texture for surface soils (0–5 cm, n = 144) of an arable field in Bangalore (India) and (2) to test and optimize different calibration strategies for GA‐PLSR for an improved estimation of soil properties. PLSR was useful for an estimation of SOC, N, sand and clay. In the cross‐validation (n = 96), accuracies of estimated soil properties generally decreased in the order GA‐PLSR > SVMR > PLSR. However, the order of estimation accuracies for the random validation sample (n = 48) changed to SVMR > GA‐PLSR > PLSR for SOC, N, pH, and CEC, whereas for clay the order changed to SVMR > PLSR > GA‐PLSR. A sequential procedure, which used the most frequently selected wavelengths of the GA‐PLSR runs, proved to be useful for an improved estimation of SOC and N. Overall, SVMR especially improved estimations of SOC and clay, whereas GA‐PLSR was particularly useful for SOC and N and it was the only approach which successfully estimated CEC in cross‐validation and validation.     

An avenue for replacement of chemical fertilization under rice-rice cropping pattern: Sustaining soil health and organic C pool via MSW-based vermicomposts

ABSTRACT

The impacts of Municipal solid wastes (MSW) vermicompost (VC) on soil-crop interface have rarely been studied. Hence, the influence of vermicompost on soil health and soil organic C (SOC) dynamics was assessed under intensive rice–rice cropping system. A 20–40% of the recommended N-fertilization was substituted by MSW-vermicompost for the crop. SOC storage, degree of humification, humic acid C, and fulvic acid C in soil gradually increased by 55–60% under NPK₆₀ + VC and NPK₈₀ + VC treatments in 2 years. The improvement in N-mineralization was spectacular in NPK₆₀ + VC (2.79 folds) and NPK₈₀ + VC (2.25 folds) treated soil. The carbon pool management index in soil was greatest under NPK₆₀ + VC (2.1) treatment followed by NPK₁₀₀ + VC (1.96) and NPK₈₀ + VC (1.87) treatments. Moreover, the crop biomass and grain yield increased under VC treatments. The correlation and regression statistics revealed that rice production was enhanced due to improvement of SOC pool and humified carbon fractions in soil. Finally, we recorded highest benefit-cost ratio under NPK₆₀ + VC (benefit/cost 5.55) followed by NPK₈₀ + VC (benefit/cost 5.44). The study indicated that MSW-vermicompost sustained soil microbial health and SOC balance, which significantly correlated with rice production.     

How environmental and vegetation factors affect spatial patterns of soil carbon and nitrogen in a subtropical mixed forest in Central China

Purpose

Soil properties are highly heterogeneous in forest ecosystems, which poses difficulties in estimating soil carbon (C) and nitrogen (N) pools. However, little is known about the relative contributions of environmental factors and vegetation to spatial variations in soil C and N, especially in highly diverse mixed forests. Here, we examined the spatial variations of soil organic carbon (SOC) and total nitrogen (TN) in a subtropical mixed forest in central China, and then quantified the main drivers.

Materials and methods

Soil samples (n = 972) were collected from a 25-ha forest dynamic plot in Badagonshan Nature Reserve, central China. All trees with diameter at breast height (DBH) ≥1 cm and topography data in the plot were surveyed in detail. Geostatistical analyses were used to characterize the spatial variability of SOC and TN, while variation partitioning combined with Mantel’s test were used to quantify the relative contribution of each type of factors.

Results and discussion

Both surface soil (0–10 cm) and subsurface soil (10–30 cm) exhibited moderate spatial autocorrelation with explainable fractions ranged from 31 to 47 %. The highest contribution to SOC and TN variation came from soil variables (including soil pH and available phosphorus), followed by vegetation and topographic variables. Although the effect of topography was weak, Mantel’s test still showed a significant relationship between topography and SOC. Strong interactions among these variables were discovered. Compared with surface soil, the explanatory power of environmental variables was much lower for subsurface soil.

Conclusions

The differences in relative contributions between surface and subsurface soils suggest that the dominating ecological process are likely different in the two soil depths. The large unexplained variation emphasized the importance of fine-scale variations and ecological processes. The large variations in soil C and N and their controlling mechanisms should be taken into account when evaluating how forest managements may affect C and N cycles.

     

Soybean Residue Effect on Carbon Fractions and Gaseous Nitrogen Losses at Contrasting Moisture Contents

Most published studies related to crop effects on denitrification are not continuous and are based on the growing period. The objective of this work was to evaluate the effect of different amounts of soybean stubble, under different soil moisture contents, on gaseous nitrogen (N) losses by denitrification from an agricultural soil. The following soil moisture treatments were reached by adding distilled water to soil cores of a typic Hapludoll: 50 and 100% of water‐filled porosity space (WFPS). Residue treatments included no application of residues, amendment with 2600 kg ha^?1 of soybean residues, and amendment with 5200 kg ha^?1 of soybean residues. Cumulative nitrous oxide + dinitrogen (N₂O + N₂) emissions displayed great variability, ranging between 0 and 581.91 µg N kg^?1, which represented 0 to 3.93% of the N residue applied. Under 50% WFPS moisture conditions, statistical differences in cumulative N₂O + N₂ emissions between residue treatments were not detected (p = 0.21), whereas at saturation conditions, cumulative N₂O + N₂ emissions decreased with the application of increasing amounts of soybean residues (p = 0.017). Daily and cumulative N₂O + N₂ emissions significantly increased as soil moisture increased, except at soils amended with 5200 kg ha^?1 of soybean residues; this lack of statistical difference was probably due to the immobilization of native mineral N. Under 50% WFPS soil moisture contents, aeration seemed to be the main factor controlling redox conditions, limiting the denitrification process, and preventing differences in N emissions between residue treatments. The application of soybean residues to saturated soils notably decreased N₂O + N₂ emissions by denitrification through a strong mineral N immobilization into organic and nondenitrifiable forms.