Water-extractable Organic C Research Articles

Beyond conventional farming: exploring the benefits of planting basins with manure on soil quality as reflected in labile organic carbon and nitrogen indicators in Kenya

PurposeSoil quality is critical for plant growth and ecosystem sustainability. Understanding the indicators that indicate soil quality is, therefore, crucial. Labile organic carbon (C) and nitrogen (N) are important components of soil functions that impact productivity and environmental stability. This study aimed to explore the sensitivity of different labile C and N fractions, including water extractable organic C (WEOC) and total N (WEON) and hot water extractable organic C (HWEOC) and total N (HWEON), to planting basins with manure compared to conventional farming practices.Methods and methodsSoil samples were drawn from 0–10 and 10–20 cm soil depths at 12 sites in Makueni County, Kenya. Samples were analysed for differences in WEOC, HWEOC, WEON, and HWEON between planting basins with manure and conventional farmer practices. We also assessed the correlations of the different labile C and N pools with other soil chemical properties linked to soil quality.Results and discussionsThe results showed that planting basins with manure significantly increased mean WEOC (171.53 µg g−1), HWEOC (353.62 µg g−1), WEON (26.60 µg g−1), and HWEON (26.39 µg g−1) compared to those of conventional farming practices (p < 0.05). WEOC was positively correlated with WEON and δ15N (p < 0.001) at the 0–10 cm soil layer suggesting that extractable organic matter can be used as an index for soil health and nutrient content.ConclusionThe findings suggest that cold water and hot water extractable C and N are sensitive indicators of the effects of different land management practices on soil quality. The results further demonstrate that planting basins with manure can improve soil quality by increasing labile C and N pools.

Impact of PVC microplastics on soil chemical and microbiological parameters

Microplastics (MPs) are emerging pollutants whose occurrence is a global problem in natural ecosystems including soil. Among MPs, polyvinyl chloride (PVC) is a well-known polymer with remarkable resistance to degradation, and because its recalcitrant nature serious environmental concerns are created during manufacturing and waste disposal.The effect of PVC (0.021% w/w) on chemical and microbial parameters of an agricultural soil was tested by a microcosm experiment at different incubation times (from 3 to 360 days). Among chemical parameters, soil CO2 emission, fluorescein diacetate (FDA) activity, total organic C (TOC), total N, water extractable organic C (WEOC), water extractable N (WEN) and SUVA254 were considered, while the structure of soil microbial communities was studied at different taxonomic levels (phylum and genus) by sequencing bacterial 16S and fungal ITS2 rDNA (Illumina MiSeq).Although some fluctuations were found, chemical and microbiological parameters exhibited some significant trends. Significant (p < 0.05) variations of soil CO2 emission, FDA hydrolysis, TOC, WEOC and WEN were found in PVC-treated soils over different incubation times. Considering the structure of soil microbial communities, the presence of PVC significantly (p < 0.05) affected the abundances of specific bacterial and fungal taxa: Candidatus_Saccharibacteria, Proteobacteria, Actinobacteria, Acidobacteria and Bacteroides among bacteria, and Basidiomycota, Mortierellomycota and Ascomycota among fungi.After one year of experiment, a reduction of the number and the dimensions of PVC was detected supposing a possible role of microorganisms on PVC degradation. The abundance of both bacterial and fungal taxa at phylum and genus level was also affected by PVC, suggesting that the impact of this polymer could be taxa-dependent.

Total and labile pools of organic carbon in relation to soil biological properties under contrasting land-use systems in a dry mountainous region

This study evaluated the effect of contrasting land-use systems on changes in pools of total organic carbon (TOC) and its labile fractions, and identified the sensitivity of soil properties as a minimum data set (MDS) for assessing soil quality change in a dry, mountainous Himalayan region in India. The soils under mono (barley/mustard/oats)- and double (barley-buckwheat/barley-turnip)-cropping systems had significantly (p < .05) lower TOC by ∼34 and 20%, while the less labile C (Fract. 3) concentration was lower by ∼46 and 48%, respectively, compared with the agro-forestry (popular/willow). The stable C pool (Fract. 3 + Fract. 4) comprised ∼74, 77, 86, 74 and 73% of TOC in soils under mono-cropping, double-cropping, agro-forestry, orchards and vegetable crops, respectively. Land-use significantly impacts the sensitivity of labile C fractions, viz. water extractable organic C (WEOC), microbial biomass C (MBC) and organic C fractions of variable oxidizability (i.e. Fract. 1, Fract. 2, Fract. 3 and Fract. 4). The sensitivity analysis showed a change of ∼21.5–56.2% in the TOC pool, with the highest change for soils under vegetable crops and the lowest for double-cropping. Soil protein exhibited a significant relationship with TOC and its fractions, enzymatic and biochemical properties, and soils’ fine fraction (silt and clay). The stable C pool exhibited a significant linear relationship with soils’ finer fraction (silt = 0.82**; clay = 0.78**, p < .01), indicating that the mineral matrix had a profound influence on C stabilization in soils. The land-use systems with higher soil total glomalin (TG) content had higher moisture retention capacity and stable C pool. The principal component analysis (PCA) identified TG, Fract. 3 and available-K as most important soil quality indicators for discriminating change in soil health in a cold, dry Himalayan region.

Black Soldier Fly Diet Impacts Soil Greenhouse Gas Emissions From Frass Applied as Fertilizer

Increased global production of animal-based protein results in high greenhouse gas (GHG) emissions and other adverse consequences for human and planetary health. Recently, commercial insect rearing has been claimed a more sustainable source of animal protein. However, this system also leaves residues called frass, which—depending on the insect diet—is rich in carbon (C) and nitrogen (N), and could thus be used as fertilizer in agriculture. The impact of this kind of fertilizer on soil GHG emissions is yet unknown. Therefore, we investigated the effect of black soldier fly (Hermetia illucens L.) frass derived from a carbohydrate (Carb-) or a protein (Prot-) based diet applied at two different application rates to an arable soil on C and N fluxes and microbial properties in a 40-day incubation experiment. CO2, N2O, NO, N2, CH4, water extractable organic C (WEOC), and inorganic N were continuously measured quantitatively. At the end of the incubation, microbial biomass (MB), stoichiometry, community composition, and abundance of functional genes were assessed. Along with a strong increase in WEOC and CO2, Carb-frass caused strong initial N2O emissions associated with high N and C availability. In contrast, Prot-frass showed lower CO2 emissions and N2O release, although soil nitrate levels were higher. At the end of incubation, MB was significantly increased, which was more pronounced following Carb-frass as compared to Prot-frass application, and at higher amendment rates. Fungal abundance increased most from both frass types with an even stronger response at higher application rates, whereas bacterial abundance rose following Carb-frass as compared to Prot-application. Abundance of functional genes related to ammonia-oxidizing bacteria and archaea were enhanced by high frass application but did not clearly differ between frass types. C use efficiency of microorganisms, as revealed by the metabolic quotient, was most strongly reduced in the high Prot-frass application rate. Overall, insect diet influenced available C and N in frass and thus affected mineralization dynamics, GHG emissions, and microbial growth. Overall, emissions were very high undermining the potential environmental benefit of insect based protein production and calling for more detailed analyses before frass is widely applied in agriculture.

Evaluating the Effects of Biochar with Farmyard Manure under Optimal Mineral Fertilizing on Tomato Growth, Soil Organic C and Biochemical Quality in a Low Fertility Soil

Biochar amendments are widely recognized to improve crop productivity and soil biogeochemical quality, however, their effects on vegetable crops are less studied. This pot study investigated the effects of cotton stick, corncob and rice straw biochars alone and with farmyard manure (FYM) on tomato growth, soil physico–chemical and biological characteristics, soil organic carbon (SOC) content and amount of soil nutrients under recommended mineral fertilizer conditions in a nutrient-depleted alkaline soil. Biochars were applied at 0, 1.5 and 3% (w/w, basis) rates and FYM was added at 0 and 30 t ha−1 rates. Biochars were developed at 450 °C pyrolysis temperature and varied in total organic C, nitrogen (N), phosphorus (P) and potassium (K) contents. The results showed that biochars, their amounts and FYM significantly improved tomato growth which varied strongly among the biochar types, amounts and FYM. With FYM, the addition of 3% corncob biochar resulted in the highest total chlorophyll contents (9.55 ug g−1), shoot (76.1 cm) and root lengths (44.7 cm), and biomass production. Biochars with and without FYM significantly increased soil pH, electrical conductivity (EC) and cation exchange capacity (CEC). The soil basal respiration increased with biochar for all biochars but not consistently after FYM addition. The water-extractable organic C (WEOC) and soil organic C (SOC) contents increased significantly with biochar amount and FYM, with the highest SOC found in the soil that received 3% corncob biochar with FYM. Microbial biomass C (MBC), N (MBN) and P (MBP) were the highest in corncob biochar treated soils followed by cotton stick and rice straw biochars. The addition of 3% biochars along with FYM also showed significant positive effects on soil mineral N, P and K contents. The addition of 3% corncob biochar with and without FYM always resulted in higher soil N, P and K contents at the 3% rate. The results further revealed that the positive effects of biochars on above-ground plant responses were primarily due to the improvements in below-ground soil properties, nutrients’ availability and SOC; however, these effects varied strongly between biochar types. Our study concludes that various biochars can enhance tomato production, soil biochemical quality and SOC in nutrient poor soil under greenhouse conditions. However, we emphasize that these findings need further investigations using long-term studies before adopting biochar for sustainable vegetable production systems.

Alterations of soil labile organic carbon fractions and biological properties under different residue-management methods with equivalent carbon input

Abstract Crop residue return to soil can greatly affect soil carbon (C) cycling. However, the differences in soil organic C (SOC) pool among different residue-management methods, including residue-biochar, are still unclear. We investigated the changes in SOC content, SOC stock, labile organic C fractions, C pool management index (CPMI), microbial abundance (bacteria, actinomycetes, and fungi), and C-cycle enzyme activities (invertase, cellulose, and β-glucosidase) after four years of applying residue and residue-biochar in a barley-cotton cropping system in Yancheng, China. Five treatments were developed based on equivalent C input: residue-biochar (RB, 3.5 Mg ha−1), residue plus microbial inoculant (RI, 5.0 Mg ha−1), residue (R, 5.0 Mg ha−1), residue mulching (RM, 5.0 Mg ha−1), and no residue or residue-biochar (CK). The results showed that SOC content, SOC stock, and the CPMI in RB were the highest among all treatments with residue and residue-biochar, being 16.3%, 8.7%, and 66.2% higher than that in CK, respectively. Regarding the labile organic C fractions, RB primarily increased the contents of particulate organic C (POC), light fraction organic C (LFOC), and permanganate oxidizable C (KMnO4-C), RI and R primarily increased the water-extractable organic C (WEOC) and microbial biomass C (MBC), whereas RM had a relatively small effect on all fractions. Soil microbial abundance and C-cycle enzyme activities were greatly enhanced by RI and R, particularly by RI. The principal component analysis revealed that POC, LFOC, and KMnO4-C were the major factors causing the changes in SOC, SOC stock, and CPMI. Soil microbial abundance and C-cycle enzyme activities were significantly correlated with WEOC and MBC. In conclusion, applying residue-biochar could effectively increase soil C sequestration by enhancing the POC, LFOC, and KMnO4-C contents in comparison with residue return.

Responses of Soil Carbon Pools, Enzymatic Activity, and Crop Yields to Nitrogen and Straw Incorporation in a Rice-Wheat Cropping System in North-Western India

Fertilizer-N application and straw incorporation impacts crop productivity due to changes in total organic carbon (TOC), its labile pools and the soil enzymatic activity. A field experiment was established (in 2010) to study the effect of fertilizer-N application (0, 90, 120 and 150 kg N ha-1) and rice straw (RS) incorporation (0, 5.0, 7.5 and 10 Mg ha-1) on crop yield, C input, TOC and its labile pools and soil enzymatic activity under rice (Oryza sativa L.) - wheat (Triticum aestivum L.) cropping system (RWCS) in north-western India. Data showed that fertilizer-N application and RS incorporation significantly (p&lt;0.05) increased the rice and wheat grain yield, compared with control (CK). However, the RS incorporation alone did not impact crop yields. The sustainable yield index (SYI) for wheat was significantly lower with RS incorporation alone as compared with the other treatments. For rice, SYI was significantly higher for RS7.5N120, and was non-significant compared with the RS10.0N120. Annual total C input in soil plough layer (0-15 cm) under RWCS varied between 3.34 and 9.78 Mg C ha-1, which was higher by 4.4 Mg C ha 1 yr-1 (~2.3-times) in RS10.0, compared with CK. The conjoint application of fertilizer-N and RS incorporation (RS10.0N150) significantly increased the TOC, water extractable organic C (WEOC), hot water C (HWC), microbial biomass C (MBC) and basal soil respiration (BSR) due to increased soil enzymatic activity viz. dehydrogenase activity (DHA), fluorescein diacetate (FDA) and alkaline phosphatise (Alk-P). A significant increase in potassium permanganate oxidizable C (KMnO4-C) with RS incorporation under RS10.0N150 increased the C management index (CMI), and hence the soil quality. The inter-correlations between highly weighted soil variables among different principle components (PCs) revealed that KMnO4-C in PC1, FDA activity in PC2, and BSR in PC3 with the highest correlation were the sensitive indicators for assessing soil quality in a RWCS.

Corn straw return can increase labile soil organic carbon fractions and improve water-stable aggregates in Haplic Cambisol

Corn straw return to the field is a vital agronomic practice for increasing soil organic carbon (SOC) and its labile fractions, as well as soil aggregates and organic carbon (OC) associated with water-stable aggregates (WSA). Moreover, the labile SOC fractions play an important role in OC turnover and sequestration. The aims of this study were to determine how different corn straw returning modes affect the contents of labile SOC fractions and OC associated with WSA. Corn straw was returned in the following depths: (1) on undisturbed soil surface (NTS), (2) in the 0–10 cm soil depth (MTS), (3) in the 0–20 cm soil depth (CTS), and (4) no corn straw applied (CK). After five years (2014–2018), soil was sampled in the 0–20 and 20–40 cm depths to measure the water-extractable organic C (WEOC), permanganate oxidizable C (KMnO4-C), light fraction organic C (LFOC), and WSA fractions. The results showed that compared with CK, corn straw amended soils (NTS, MTS and CTS) increased SOC content by 11.55%–16.58%, WEOC by 41.38%–51.42%, KMnO4-C and LFOC by 29.84%–34.09% and 56.68%–65.36% in the 0–40 cm soil depth. The LFOC and KMnO4-C were proved to be the most sensitive fractions to different corn straw returning modes. Compared with CK, soils amended with corn straw increased mean weight diameter by 24.24%–40.48% in the 0–20 cm soil depth. The NTS and MTS preserved more than 60.00% of OC in macro-aggregates compared with CK. No significant difference was found in corn yield across all corn straw returning modes throughout the study period, indicating that adoption of NTS and MTS would increase SOC content and improve soil structure, and would not decline crop production.

Fluorescence spectroscopy: A sensitive tool for identifying land-use and climatic region effects on the characteristics of water-extractable soil organic matter

Water-extractable organic matter (WEOM) is an indicator of active and labile pools of soil organic matter (SOM). Land-use is expected to have a major effect on SOM, yet the effects of different land-uses on WEOM are not well understood. Reported trends in the changes in water-extractable organic C (WEOC) concentrations following changes in land-use are inconsistent or even contradictory. Fluorescent WEOM components, including humic-like and proteinaceous substances, may be useful for elucidating long-term effects of different land-uses on WEOM content and composition. Therefore, the objectives of this research were (i) to characterize SOM and WEOM in soils from different climatic regions that were subjected to different land-uses and (ii) to compare the sensitivity of fluorescent components in WEOM to that of other SOM/WEOM attributes for evaluating the impact of land-use and climatic region on SOM. Two soil types from different climatic regions (Mediterranean and semi-arid) that were collected from land subjected to different uses (field crops, orchards and non-cultivated plots) and characterized for total soil organic C and total N. The concentration of WEOC, UV absorbance at 254 nm and excitation-emission matrices (EEMs) of fluorescence were determined in diluted solution extracts. Parallel-factor (PARAFAC) analysis was applied to identify meaningful fluorescent components of WEOM and quantify their presence. Most of the quantitative measures used for SOM and WEOM characterization demonstrated insignificant and/or inconsistent relationships with the type of land-use and climatic region. However, EEM-PARAFAC-derived fluorescent components present in soil WEOM and, specifically, their relative fractions exhibited significant associations with the climatic region and type of land-use. These results may support the use of EEM-PARAFAC fluorescent components as indicators for the assessment of soil health.

Higher biochar rate strongly reduced decomposition of soil organic matter to enhance C and N sequestration in nutrient-poor alkaline calcareous soil

Our knowledge of biochar as a tool for soil carbon (C) sequestration and soil quality in semi-arid regions remains limited in recent years. Moreover, there is a lack of understanding on the comparison between effects of biochar rates on soil organic C (SOC) and nitrogen (N) mineralization in organic C poor soils. Interactive effects of corncob-biochar (B), prepared at 450 °C pyrolysis temperature, and wheat straw (W) on soil native and fresh organic matter were investigated in sandy loam alkaline calcareous soil. The corncob-biochar was applied alone and in combination with wheat straw (WB) at rates equivalent to 0 t ha-1 (unamended control), 22.5 t ha-1 (B1, W1, W1B1), and 45 t ha-1 (B2, W2, W2B2). After treatment applications, soils were incubated under laboratory conditions for 61 days and periodic CO2 evolution rates were measured to calculate the cumulative CO2 efflux and priming effects. The lowest CO2 efflux was found in B2 treatment, whereas CO2 efflux was lower in W1B1 than in W1 treatment. Biochar alone and with wheat straw significantly reduced net C mineralization (NCM) compared to wheat straw only treatments and the effects became stronger as biochar rate increased. This was reflected in data on the priming effect, which also showed that increased biochar rates induced a stronger negative priming (− 87 to − 67%) on SOC. Biochar with wheat straw at both rates enhanced microbial biomass C (MBC) compared to the control, W1, B1, and B2 treatments. However, the W1B1 and W2B2 treatments decreased water-extractable organic C (WEOC) contents compared to W1 and W2. Microbial metabolic quotient (qCO2) and MBC/TOC ratio increased with biochar rate, suggesting more efficient utilization of labile C by microorganisms at higher biochar rate. Biochar alone and with wheat straw reduced soil mineral nitrogen (N) contents than the control treatment and resulted in net negative N mineralization or net N immobilization. Our findings indicate that biochar can reduce decomposition of soil organic matter to facilitate C and N sequestration in low organic C soils. However, our study also suggests the co-application of biochar and organic amendments/residues as labile C source to induce co-metabolism to improve microbial functions and C utilization efficiency in soils that are deficient in organic C.

Sandy Soil Amended with Clay Soil: Effect of Clay Soil Properties on Soil Respiration, Microbial Biomass, and Water Extractable Organic C

The low organic C (OC) sequestration and low water and nutrient holding capacities of sandy soils can be improved by amendment with clay soils. However, the effect of clay soils may depend on clay soil properties, including cation exchange capacity (CEC) and clay and OC contents. Two experiments were carried out. In the first experiment, sandy soil was mixed with two high clay content soils (Sodosol and Vertosol) at 10% and 40% (w/w). The soil was left unamended or amended with wheat straw, to assess if the effect of clay soil addition was influenced by OC addition. In the second experiment, sandy soil was mixed with two low-clay content soils which differed in CEC without wheat straw amendment. In experiment 1, compared with sandy soil alone, amendment with Sodosol (low OC) reduced cumulative respiration more strongly than addition of Vertosol. Without wheat straw addition, both clay soils increased microbial biomass C (MBC), whereas with wheat straw, they reduced microbial biomass C (MBC) compared with sandy soil alone. Sodosol amendment reduced water-extractable OC (WEOC) without wheat straw but increased it with wheat straw. In experiment 2, only the high CEC clay soil reduced cumulative respiration, MBC and WEOC compared with sandy soil alone. Clay soil addition to sandy soil will result in the greatest OC sequestration if the added soil has high clay but low OC content and its CEC is high.

Water extractable organic C and total N: The most sensitive indicator of soil labile C and N pools in response to the prescribed burning in a suburban natural forest of subtropical Australia

Soil labile carbon (C) and nitrogen (N) pools are important indicators of soil fertility that are sensitive to environmental disturbance. However, few studies have periodically examined the long-term dynamics of soil labile C and N pools following prescribed burning and their implications on nutrient cycling in forest ecosystems. In this study, 0–5, 5–10, and 10–20 cm soil depths were sampled multiple times, spaning six years across a series of prescribed burning sites in a eucalypt natural forest of subtropical Australia. Water extractable organic C (WEOC) and total N (WETN), hot water extractable organic C (HWEOC) and total N (HWETN), microbial biomass C (MBC) and N (MBN) were analysed and correlated with the post-fire recovery time and other major environmental factors. Prescribed burning had long-lasting impact on WEOC and WETN for nearly 12 years following the burning. The lengthy recovery time was associated with the slow recovery in the forest floor of organic layers, which significantly diminished upon the burning. This was longer than the proposed 8–10 year burning cycle in Toohey Forest, and necessitated extending the burning cycle or reducing burning intensity to preserve the soil fertility. The WEOC and WETN were the most sensitive indictors of burning disturbances on soil labile C and N pools in this forest ecosystem. The MBC & MBN, and HWEOC & HWETN, however, did not show consistent patterns of recovery. More studies are needed to differentiate between the burning impacts and the seasonal impacts in the context of intensifying climate change and urban developments.

Influences of Hillslope Biogeochemistry on Anaerobic Soil Organic Matter Decomposition in a Tundra Watershed

AbstractWe investigated rates and controls on greenhouse gas (CO2 and CH4) production in two contrasting water‐saturated tundra soils within a permafrost‐affected watershed near Nome, Alaska, United States. Three years of field sample analysis have shown that soil from a fen‐like area in the toeslope of the watershed had higher pH and higher porewater ion concentrations than soil collected from a bog‐like peat plateau at the top of the hillslope. The influence of these contrasting geochemical and topographic environments on CO2 and CH4 production was tested in soil microcosms by incubating both the organic‐ and mineral‐layer soils anaerobically for 55 days. Nitrogen (as NH4Cl) was added to half of the microcosms to test potential effects of N limitation on microbial greenhouse gas production. We found that the organic toeslope soils produced more CO2 and CH4, fueled by higher pH and higher concentrations of water‐extractable organic C (WEOC). Our results also indicate N limitation on CO2 production in the peat plateau soils but not the toeslope soils. Together these results suggest that the weathering and leaching of ions and nutrients from tundra hillslopes can increase the rate of anaerobic soil organic matter decomposition in downslope soils by (1) increasing the pH of soil porewater; (2) providing bioavailable WEOC and fermentation products such as acetate; and (3) relieving microbial N limitation through nutrient runoff. We conclude that the soil geochemistry as mediated by landscape position is an important factor influencing the rate and magnitude of greenhouse gas production in tundra soils.

Maize root and shoot litter quality controls short-term CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O emissions and bacterial community structure of arable soil

Abstract. Chemical composition of root and shoot litter controls decomposition and, subsequently, C availability for biological nitrogen transformation processes in soils. While aboveground plant residues have been proven to increase N2O emissions, studies on root litter effects are scarce. This study aimed (1) to evaluate how fresh maize root litter affects N2O emissions compared to fresh maize shoot litter, (2) to assess whether N2O emissions are related to the interaction of C and N mineralization from soil and litter, and (3) to analyze changes in soil microbial community structures related to litter input and N2O emissions. To obtain root and shoot litter, maize plants (Zea mays L.) were cultivated with two N fertilizer levels in a greenhouse and harvested. A two-factorial 22 d laboratory incubation experiment was set up with soil from both N levels (N1, N2) and three litter addition treatments (control, root, root + shoot). We measured CO2 and N2O fluxes, analyzed soil mineral N and water-extractable organic C (WEOC) concentrations, and determined quality parameters of maize litter. Bacterial community structures were analyzed using 16S rRNA gene sequencing. Maize litter quality controlled NO3- and WEOC availability and decomposition-related CO2 emissions. Emissions induced by maize root litter remained low, while high bioavailability of maize shoot litter strongly increased CO2 and N2O emissions when both root and shoot litter were added. We identified a strong positive correlation between cumulative CO2 and N2O emissions, supporting our hypothesis that litter quality affects denitrification by creating plant-litter-associated anaerobic microsites. The interdependency of C and N availability was validated by analyses of regression. Moreover, there was a strong positive interaction between soil NO3- and WEOC concentration resulting in much higher N2O emissions, when both NO3- and WEOC were available. A significant correlation was observed between total CO2 and N2O emissions, the soil bacterial community composition, and the litter level, showing a clear separation of root + shoot samples of all remaining samples. Bacterial diversity decreased with higher N level and higher input of easily available C. Altogether, changes in bacterial community structure reflected degradability of maize litter with easily degradable C from maize shoot litter favoring fast-growing C-cycling and N-reducing bacteria of the phyla Actinobacteria, Chloroflexi, Firmicutes, and Proteobacteria. In conclusion, litter quality is a major driver of N2O and CO2 emissions from crop residues, especially when soil mineral N is limited.

Quantifying soil quality in a horticultural-cover cropping system

Understanding and interpretation of soil quality is crucial to strategize land management practices for sustaining agricultural productivity while mitigating environmental degradation. A medium-term cover crop (CC) trial, established in 2007 and repeated in 2008, at Ridgetown, Ontario was used to evaluate the comprehensive assessment of soil health (CASH), to develop a weighted soil quality index (WSQI), and to compare scores of CASH, Haney soil health test (HSHT), and WSQI in surface soil (15 cm) in 2015 and 2016 (herein referred to as site-years). Out of 25 potential soil quality indicators, 19 soil indicators were responsive to CC treatments and represented a total dataset. Using principal component analysis (PCA) of the total dataset, a minimum dataset (five indicators; pH, organic matter (OM), Solvita labile amino N (SLAN), Solvita CO2-burst, and water extractable organic C (WEOC)) was identified to calculate a WSQI. The WSQI and CASH scores were equivalent in detecting CC treatment differences in site-year 2015, but CASH detected greater magnitude (12.1%) of treatment differences than WSQI (5.7%) in site-year 2016. Cereal rye and a mixture of oilseed radish and rye (OSR + Rye) led to greater soil quality values than other tested CCs. Our results indicate the potential of CCs in significantly improving soil quality in the medium-term (6 years). This study is the first independent evaluation of CASH in a horticultural system in a humid, temperate climate, and first study to compare WSQI with commercial soil quality tests. Even though CASH and WSQI differentiated between CC treatments for soil quality evaluation in both site-years, we recommend WSQI as a valuable and practical tool (lesser number of indicators; five vs. 15), for quantifying soil quality in similar production regions and climatic conditions.

Soil Health and Soil Fertility Assessment by the Haney Soil Health Test in an Agricultural Soil in West Tennessee

ABSTRACTThe Haney Soil Health Test (HSHT) is a recent approach to quantify soil health by focusing mostly on soil biology. It uses a new extractant (H3A) for the extraction of plant available nutrients, a new method of soil respiration measurement (24-hr CO2 burst) using Solvita® gel system, and a new approach in determining bioavailable carbon (C) and nitrogen (N) namely water extractable organic C (WEOC) and water extractable organic N (WEON). A soil health score is calculated by combining Solvita® respiration, WEOC, and WEON data. The objective of this study was to test the feasibility of HSHT to detect the management-induced changes in soil nutrient levels and soil health in the production systems of west Tennessee. We tested soils collected from a cover crop field trial established in 2013 on a no-till corn (Zea mays L.) – soybean [Glycine max (L.) Merr.] rotation. Compared to Mehlich-1 and Mehlich-3, H3A extracted the lowest amount of soil extractable phosphorus, potassium, calcium and magnesium. Neither the soil health score nor its component parameters showed significant differences between cover crop treatments and the control. In addition, the Solvita® CO2 data did not provide a reliable estimation of potentially mineralizable N. Overall, the HSHT did not detect differences in soil health due to cover cropping in west Tennessee. We conclude that HSHT, though a promising concept due to its focus on linking soil biology with soil fertility and soil health, may need extensive field evaluation and refinement in contrasting soils and climates across the US.

Effects of changed litter inputs on soil labile carbon and nitrogen pools in a eucalyptus-dominated forest of southeast Queensland, Australia

The quantity and quality of litter inputs to forest soils are likely to be changed as a result of the climate change and human disturbances. However, the effects of changed litter inputs on soil labile carbon (C) and nitrogen (N) pools still remain unclear. A 15-month in situ field experiment was conducted within both high and low litter quality site in a eucalyptus-dominated native forest of Queensland, Australia. Three rates of litter inputs were applied, including (i) no litter (NL); (ii) single litter (SL), representing the average condition of the surrounding forest floor; and (iii) double litter (DL). Water-extractable organic C (WEOC) and total N (WETN), hot water-extractable organic C (HWEOC) and total N (HWETN), microbial biomass C (MBC), and N (MBN) were analyzed in the 0–5-cm soil layer seasonally. Litter input rates had no significant effects on litter decomposition at both sites (P > 0.05). After 15-month of decomposition, mean litter mass loss was 46.3% and 31.2% at the HQ and LQ sites, respectively. Changed litter quantity had no significant effects on any of the soil labile C and N pools, regardless of litter quality. However, soil labile C and N pools significantly varied with sampling times, and the samples of different sampling times were clearly separated at both sites according to the redundancy analysis (RDA). WEOC peaked in summer, declined in autumn and winter, and increased again in spring, while the concentrations of HWEOC and MBC peaked in the winter period. The seasonal trends of MBN were opposite to the trends of WETN, which might be due to the temporal partitioning of N between plants and microbes. The findings indicated that soil labile C and N pools in the eucalyptus-dominated forest of subtropical Australia were resistant to a short-term change in aboveground litter inputs. Future research should expand on these findings by keeping observing over a longer time period and considering the influence of changed belowground litter inputs.

Biotic versus Abiotic Controls on Bioavailable Soil Organic Carbon

Processes controlling microbial access to soil organic matter are critical for soil nutrient cycling and C stabilization. The bioavailability of soil organic matter partly depends on the rate that substrates become water-soluble, which is determined by some combination of biological, biochemical, and purely abiotic processes. Our goal was to unravel these biotic and abiotic processes to better understand mechanisms controlling the dynamics of bioavailable soil organic carbon (SOC). We sampled soils in a California annual grassland from manipulated plots with and without plants to help distinguish bioavailable SOC generated from mineral-associated organic matter versus from plant detritus (i.e., the “light fraction”). In the laboratory, soils were incubated for 8 months under all possible combinations of three levels of moisture and two levels of microbial biomass using continuous chloroform sterilization. We measured cumulative carbon dioxide (CO2) production and the net change in soil water-extractable organic C (WEOC) to quantify C that was accessed biologically or biochemically. Under the driest conditions, microbes appeared to primarily access WEOC from recent plant C, with the other half of CO2 production explained by extracellular processes. These results suggest that dry, uncolonized conditions promote the adsorption of WEOC onto mineral surfaces. Under wetter conditions, microbial access increased by two orders of magnitude, with a large concomitant decrease in WEOC, particularly in soils without plant inputs from the previous growing season. The largest increase in WEOC occurred in wet sterilized soil, perhaps because exoenzymes and desorption continued solubilizing C but without microbial consumption. A similar amount of WEOC accumulated in wet sterilized soil whether plants were present or not, suggesting that desorption of mineral-associated C was the abiotic WEOC source. Based on these results, we hypothesize that dry-live and wet-uncolonized soil microsites are sources of bioavailable SOC, whereas wet-live and dry-uncolonized microsites are sinks.

Increasing molecular structural complexity and decreasing nitrogen availability depress the mineralization of organic matter in subtropical forest soils

The degradation of soil organic matter (SOM) is determined by its compositional and structural properties, but the properties of the remaining SOM can be modified during the mineralization process. In this study, 12 soils (Ultisols, Alfisols and Inceptisols) from subtropical forests were incubated under laboratory conditions (25 °C, aeration) for 90 days. The mechanisms underlying the changes of these properties were investigated by analyzing pre- and post-incubation soils using UV absorption, fluorescence spectroscopy and acid hydrolysis methods. The turnover rate of SOM decreased with increasing soil depth. The amount of CO2 released ranged from 90 to 620 mg CO2-C kg−1, representing c. 0.9–3.1% of soil organic C (SOC). The proportion of nonhydrolyzable (recalcitrant) N in total N (RN) increased with soil depth, whereas it was not apparent in relation to the proportion of nonhydrolyzable (recalcitrant) C in total C (RC). Furthermore, a negative correlation was observed between SOM mineralization and RN (P < 0.05) rather than RC, implying that the availability of N was a limiting factor in SOM decomposition when compared with C. The decrease in E2-to-E4 ratio and fluorescence intensity (FI) of water-extractable organic C (WEOC) with increasing soil depth suggested that WEOC in the deeper horizons was probably rich in electron-withdrawing groups (COOH) but poor in electron-donating groups (OH, NH2), and it was composed of components with larger molecular size, higher humification degree and structural complexity. These could be responsible for the lower SOM mineralization rates in deeper horizons. After 90 days of incubation, a blue shift of wavelengths in the synchrotron-scan spectra, and an obvious increase of FI values, together with the increase in E2-to-E4 ratio of WEOC implied a reduction in molecular size and the degree of aromatic condensation and humification, resulting in a shift to less complicated structures of WEOC.

Corncob-derived biochar decelerates mineralization of native and added organic matter (AOM) in organic matter depleted alkaline soil

The understanding of the carbon (C) and nitrogen (N) mineralization is essential to mitigate the climate change and improve the soil quality for agricultural sustainability. An incubation study was carried out to investigate the effects of corncob-derived biochar with and without the corncob residues, as added organic matter (AOM), on C and N mineralization in an organic C deficient alkaline soil. The study used biochar produced from corncobs by pyrolysing the biomass at 400°C and consisted of four treatments: unamended control soil, soil +2% biochar (45tonsha−1), soil +2% corncob residue (45tonsha−1) and soil +1% corncob residue +1% biochar (22.5tonsha−1 each of biochar and residue). Soil C mineralization was quantified by measuring soil respiration periodically throughout the experiment over the 54d incubation period. Soils were analyzed for microbial biomass C (MBC), water extractable organic C (WEOC) and N mineralization. We also calculated microbial metabolic quotient (qCO2), net C mineralization (NCM), net CO2 efflux and priming effect values. The results revealed that only a very little fraction of biochar was respired whereas a high proportion of corncob was decomposed. Biochar addition suppressed decomposition of native and AOM by 16.31% and 30.13% respectively through negative priming effect (−40.01%). Compared to the corncob residue alone, addition of biochar with corncob residue significantly increased MBC but decreased WEOC, qCO2, NCM and net CO2 efflux. Moreover, residue significantly increased the net CO2 efflux, NCM and qCO2 and, at the same time, decreased net N mineralization. Extractable NH4+-N, NO3−-N and mineral-N contents were the highest in the biochar and biochar+residue treatments. In addition, biochar alone and in combination with residue resulted in higher negative net N mineralization/immobilization. Enhanced MBC contents and reduced qCO2 indicated efficient C utilization by microorganisms. The results of this study conclude that biochar could decrease C mineralization but enhanced microbial C use efficiency and, therefore, offer an important management strategy to improve C sequestration in nutrient and organic C deficient alkaline soil. The results also imply that the amendment of corncob residue with biochar had a potential to reduce the rate of the soil N leaching and N2O emission by N immobilization and sorption to biochar.