Variations in soil microbial biomass and crop roots due to differing resource quality inputs in a tropical dryland agroecosystem

The influence of exogenous organic inputs on soil microbial biomass dynamics and crop root biomass was studied through two annual cycles in rice-barley rotation in a tropical dryland agroecosystem. The treatments involved addition of equivalent amount of N (80 kg N ha⁻¹) through chemical fertilizer and three organic inputs at the beginning of each annual cycle: Sesbania shoot (high-quality resource, C:N 16, lignin:N 3.2, polyphenol+lignin:N 4.2), wheat straw (low-quality resource, C:N 82, lignin:N 34.8, polyphenol+lignin:N 36.8) and Sesbania+wheat straw (high-and low-quality resources combined), besides control. The decomposition rates of various inputs and crop roots were determined in field conditions by mass loss method. Sesbania (decay constant, k=0.028) decomposed much faster than wheat straw (k=0.0025); decomposition rate of Sesbania+wheat straw was twice as fast compared to wheat straw. On average, soil microbial biomass levels were: rice period, Sesbania?Sesbania+wheat straw>wheat straw?fertilizer; barley period, Sesbania+wheat straw>Sesbania?wheat straw?fertilizer; summer fallow, Sesbania+wheat straw>Sesbania>wheat straw?fertilizer. Soil microbial biomass increased through rice and barley crop periods to summer fallow; however, in Sesbania shoot application a strong peak was obtained during rice crop period. In both crops soil microbial biomass C and N decreased distinctly from seedling to grain-forming stages, and then increased to the maximum at crop maturity. Crop roots, however, showed reverse trend through the cropping period, suggesting strong competition between microbial biomass and crop roots for available nutrients. It is concluded that both resource quality and crop roots had distinct effect on soil microbial biomass and combined application of Sesbania shoot and wheat straw was most effective in sustained build up of microbial biomass through the annual cycle.     

Microbiological parameters as indicators of soil quality under various soil management and crop rotation systems in southern Brazil

The objective of this work was to identify soil parameters potentially useful to monitor soil quality under different soil management and crop rotation systems. Microbiological and chemical parameters were evaluated in a field experiment in the State of Paraná, southern Brazil, in response to soil management [no-tillage (NT) and conventional tillage (CT)] and crop rotation [including grain (soybean, S; maize, M; wheat, W) and legume (lupin, L.) and non-legume (oat, O) covers] systems. Three crop rotation systems were evaluated: (1) (O/M/O/S/W/S/L/M/O/S), (2) (O/S/L/M/O/S/W/S/L/M), and (3) (O/S/W/S/L/M/O/M/W/M), and soil parameters were monitored after the fifth year. Before ploughing, CO₂-emission rates were similar in NT and CT soils, but plough increased it by an average of 57%. Carbon dioxide emission was 13% higher with lupin residues than with wheat straw; decomposition rates were rapid with both soil management systems. Amounts of microbial biomass carbon and nitrogen (MB-C and MB-N, respectively) were 80 and 104% higher in NT than in CT, respectively; however, in general these parameters were not affected by crop rotation. Efficiency of the microbial community was significantly higher in NT: metabolic quotient (qCO₂) was 55% lower than in CT. Soluble C and N levels were 37 and 24% greater in NT than in CT, respectively, with no effects of crop rotation. Furthermore, ratios of soluble C and N contents to MB-C and MB-N were consistently lower in NT, indicating higher immobilization of C and N per unit of MB. The decrease in qCO₂ and the increase in MB-C under NT allowed enhancements in soil C stocks, such that in the 0–40 cm profile, a gain of 2500 kg of C ha⁻¹ was observed in relation to CT. Carbon stocks also varied with crop rotation, with net changes at 0–40 cm of 726, 1167 and −394 kg C ha⁻¹ year, in rotations 1, 2 and 3, respectively. Similar results were obtained for the N stocks, with 410 kg N ha⁻¹ gained in NT, while crop rotations 1, 2 and 3 accumulated 71, 137 and 37 kg of N ha⁻¹ year⁻¹, respectively. On average, microbial biomass corresponded to 2.4 and 1.7% of the total soil C, and 5.2 and 3.2% of the N in NT and CT systems, respectively. Soil management was the main factor affecting soil C and N levels, but enhancement also resulted from the ratios of legumes and non-legumes in the rotations. The results emphasize the importance of microorganisms as reservoirs of C and N in tropical soils. Furthermore, the parameters associated with microbiological activity were more responsive to soil management and crop rotation effects than were total stocks of C and N, demonstrating their usefulness as indicators of soil quality in the tropics.     

Carbon budget and seasonal carbon dioxide emission from a central Ohio Luvisol as influenced by wheat residue amendment

Enhancement of soil organic carbon (SOC) stocks through mulching has been proposed, and although this practice can alter several soil properties, its impact on the temporal variability of carbon dioxide (CO₂) emission from soils has not been widely investigated. To that end, we monitored CO₂ fluxes from a central Ohio Luvisol (fine, mixed, mesic Aeric Ochraqualf) amended with wheat (Triticum aestivum L.) straw applied at rates of 0 (M0), 8 (M8) and 16 (M16) Mg dry matter ha⁻¹ per year and supplemented with fertilizer (244 kg N ha⁻¹ per year) or without. The experimental design was a randomized complete block design with three replications. The intensity of CO₂ emission was higher in the late winter (mean: 2.79 g CO₂-C m⁻² per day) and summer seasons (2.45 g CO₂-C m⁻² per day) and lowest in the autumn (1.34 g CO₂-C m⁻² per day). While no significant effect of N fertilization on CO₂ emission was detected, soil mulching had a significant effect on the seasonal variation of CO₂ fluxes. The percentage of annual CO₂ emitted during the winter and spring was similar across treatments (17–22%); however, 43% of the annual CO₂ loss in the M0 plots occurred during the summer as opposed to 26% in the mulch treatments. A close relationship (F=0.47X+4.45, R²=0.97, P<0.001) was found between annual CO₂ flux (F, Mg CO₂-C ha⁻¹) and residue-C input (X, Mg C ha⁻¹). Litter and undecomposed residue amounted to 0.32 and 0.67 Mg C ha⁻¹ per year in the M8 and M16 plots, respectively. After 4 years of straw application, SOC stocks (0–10 cm) were 19.6, 25.6 and 26.5 Mg C ha⁻¹ in the M0, M8 and M16 treatments, respectively. The results show that soil mulching has beneficial effect on SOC sequestration and strongly influence the temporal pattern of CO₂ emission from soils.     

小麦和玉米秸秆腐解特点及对土壤中碳、氮含量的影响

通过室内模拟培养试验,揭示了不同水分条件下小麦和玉米秸秆在土壤中的腐解特点及对土壤碳、氮含量的影响。结果表明,1)水分条件对有机物质腐解的影响较大,在32 d的培养期间,相对含水量为60%(M60)时,土壤CO2释放速率始终低于含水量80%(M80)的处理。M60条件下释放的CO2-C量占秸秆腐解过程中释放碳总量的40.1%,而M80条件下达到51.5%;M60条件下,添加秸秆土壤中有机碳含量平均提高2.24 g/kg,显著高于M80条件下的1.43 g/kg。2)添加玉米秸秆的土壤,在培养期内CO2释放速率始终高于小麦秸秆处理,CO2-C累积释放量和有机碳净增量分别为408.35 mg/pot和2.12 g/kg;而小麦秸秆处理分别仅为378.94 mg/pot和1.56 g/kg,两种秸秆混合的处理介于二者之间。3)与未添加秸秆相比,土壤中添加小麦或玉米秸秆后,土壤有机碳、微生物量碳、全氮和微生物量氮含量均显著提高,且数量上总体趋势表现为:玉米秸秆两种秸秆混合小麦秸秆。可见,适宜水分条件有利于秸秆腐解过程中秸秆中碳向无机碳方向转化,而不利于向土壤有机碳方向转化;且玉米秸秆比小麦秸秆更易腐解。秸秆在土壤中腐解对补充土壤碳、氮作用很大,可改善土壤微生物生存条件,提高土壤质量。     

Repeated CO2 pulse-labelling reveals an additional net gain of soil carbon during growth of spring wheat under free air carbon dioxide enrichment (FACE)

Rising levels of atmospheric CO₂ have often been found to increase above and belowground biomass production of C3 plants. The additional translocation of organic matter into soils by increased root mass and exudates are supposed to possibly increase C pools in terrestrial ecosystems. Corresponding investigations were mostly conducted under more or less artificial indoor conditions with disturbed soils. To overcome these limitations, we conducted a ¹⁴CO₂ pulse-labelling experiment within the German FACE project to elucidate the role of an arable crop system in carbon sequestration under elevated CO₂. We cultivated spring wheat cv. “Minaret” with usual fertilisation and ample water supply in stainless steel cylinders forced into the soil of a control and a FACE plot. Between stem elongation and beginning of ripening the plants were repeatedly pulse-labelled with ¹⁴CO₂ in the field. Soil born total CO₂ and ¹⁴CO₂ was monitored daily till harvest. Thereafter, the distribution of ¹⁴C was analysed in all plant parts, soil, soil mineral fractions and soil microbial biomass. Due to the small number of grown wheat plants (40) in each ring and the inherent low statistical power, no significant above and belowground growth effect of elevated CO₂ was detected at harvest. But in comparison to ambient conditions, 28% more ¹⁴CO₂ and 12% more total CO₂ was evolved from soil under elevated CO₂ (550 μmol CO₂ mol⁻¹). In the root-free soil 27% more residual ¹⁴C was found in the FACE soil than in the soil from the ambient ring. In soil samples from both treatments about 80% of residual ¹⁴C was found in the clay fraction and 7% in the silt fraction. Very low ¹⁴C contents in the CFE extracts of microbial biomass in the soil from both CO₂ treatments did not allow assessing their influence on this parameter. Since the calculated specific radioactivity of soil born ¹⁴CO₂ gave no indication of an accelerated priming effect in the FACE soil, we conclude that wheat plants grown under elevated CO₂ can contribute to an additional net carbon gain in soils.     

CO2 evolution and N mineralization after biogas slurry application in the field and its yield effects on spring barley

The objective of this study was to investigate the effects of biogas slurry derived from straw-rich farmyard manure on the soil microbial biomass, on the mineralization in the field and on the related crop yield. The experiment was carried out in the following four treatments: (1) fallow, (2) fallow + biogas slurry, (3) spring barley, and (4) spring barley + biogas slurry. The CO₂ evolution rate ranged between 15 and 120 mg C m⁻² h⁻¹ in both fallow treatments and showed a significant exponential relationship with the soil temperature at 5 cm depth. According to the extrapolation of the CO₂ evolution rates into amounts per hectare, approximately 200 kg C ha⁻¹ or 27% of the biogas slurry derived C were mineralized to CO₂ during a 50 days’ period to 18 June in the fallow treatment with biogas slurry. An additional amount of up to 29.5 kg inorganic N ha⁻¹ could be calculated as the sum of NH₄-N already present in biogas slurry at the time of amendment and from the amount of biogas slurry mineralized in the soil to NO₃-N. A good agreement between measured and modelled stocks of inorganic N at 0–60 cm depth was obtained after having five-fold increased soil organic C turnover compared to the default values of the model DNDC. The mineralization data are in line with an amount of up to 21 kg ha⁻¹ more N transferred by the barley plants to their aboveground biomass in biogas slurry treatment. The N not accounted for by the aboveground plant biomass could be explained by the belowground plant-derived N. CO₂ evolution from the soil surface, inorganic N content at 0–60 cm depth and N transfer into barley aboveground biomass lead apparently to similar results after the application of biogas slurry. The soil ATP content after harvest of the barley was significantly larger in the two treatments with biogas slurry, especially in the fallow treatment indicating a positive effect on the soil microbial community.     

Soil structure and organic carbon relationships following 10 years of wheat straw management in no-till

Crop residue retention is important for sequestering soil organic carbon (SOC), controlling soil erosion, and improving soil quality. Magnitude of residue management impacts on soil structural properties and SOC sequestration is, however, site specific. This study assessed long-term (10 year) impacts of three levels (0, 8, and 16 Mg ha⁻¹ on a dry matter basis) of wheat (Triticum aestivum L.) straw applied annually on SOC concentration and physical properties of the bulk soil and individual 5- to 8-mm aggregates for the 0- to 50-cm soil depth under no-till (NT) on a Crosby silt loam (fine, mixed, active, mesic Aeric Epiaqualfs) in central Ohio. This study also quantified relationships between soil properties and straw-induced changes in SOC concentration. Changes in soil properties due to straw mulching were mostly confined to the upper 5 cm of the soil. Mulching increased SOC concentration, but it did not significantly change cone index (CI) and shear strength (SHEAR). Within the upper 0–5-cm soil depth, mulching decreased bulk density (ρ_b) by 40–50%, aggregate density (ρ_agg) by 30–40%, and particle density (ρ_s) by 10–15%, and increased tensile strength (TS) of aggregates by up to 14 times as compared to unmulched soil. At the same depth, soil with mulch retained >30% more water than soil without mulch from 0 to −1500 kPa potentials. The SOC amount was 16.0 Mg ha⁻¹ under no straw, 25.3 Mg ha⁻¹ under 8 Mg ha⁻¹ straw, and 33.5 Mg ha⁻¹ under 16 Mg ha⁻¹ straw in the 0- to 10-cm depth. Below 10 cm, differences in SOC pool between mulched and unmulched soil were not significant. Overall, SOC from 0- to 50-cm depth was 82.5 Mg ha⁻¹ for unmulched soil, 94.1 Mg ha⁻¹ for 8 Mg ha⁻¹ mulch, and 104.9 Mg ha⁻¹ for 16 Mg ha⁻¹. About 33% of C added with straw over the 10-year period was sequestered in soil. This means that 2/3 of the wheat straw applied was not converted to SOC and most probably was lost as emissions of CO₂ and CH₄. The annual rate of total C accrual was 1.2 Mg ha⁻¹ in soil mulched with 8 Mg ha⁻¹ and 2.2 Mg ha⁻¹ in soil mulched with 16 Mg ha⁻¹ of straw in the 0- to 50-cm depth. The percentage of macroaggregates (>5-mm) was six times higher under 8 Mg ha⁻¹ of straw and 12 times higher under 16 Mg ha⁻¹ compared to unmulched treatments. Macroaggregates contained greater SOC than microaggregates in mulched soil. The SOC concentration explained the variability in aggregate properties by as much as 96%. Overall, long-term straw mulching increased SOC concentration and improved near-surface aggregate properties.     

Evaluation of soil quality parameters in a tropical paddy soil amended with rice residues and tree litters

Laboratory and greenhouse experiments were conducted to study the effects of applications of rice residue and Pongamia pinnata and Azadirachta indica leaf litters on biochemical properties (extraction yield of humus, composition of humus, microbial biomass carbon, activities of urease and acid phosphatase) of a lowland rice soil under flooded conditions. Bulk soil sample collected from the Mandya paddy fields was used for the green house trials and the laboratory incubation studies. The organic materials were added at three rates – zero, 25.0 g carbon kg⁻¹ (2.5% C) and 50.0 g carbon kg⁻¹ dry soil (5.0% C). Results showed that tree leaf litter and rice residue at 5.0% C rate decreased instantaneous decay constant (k), there by retarded the rate of C mineralization. Carbon contents of HA increased with the rate of C added. Study of delta–log K values and C contents of humic acids revealed that greatest molecular weight of HA was in the pongamia litter treatment, followed by neem litter and rice residue. Grain and straw yields of rice crop in the pot culture study were statistically correlated to the soil quality parameters. Neem and pongamia tree litter incorporation at 2.5% C could be considered for improving soil health and crop yields of rice under flooded conditions; however, application at higher rates significantly (P ≤ 0.05) lowered total dry matter production in rice, despite favorable soil health parameters such as humic yields, microbial biomass – C content and acid phosphatase and urease activity. Among different soil health parameters, microbial quotient was found to be more sensitive indicator of decline in soil quality.     

Spatial patterns of soil biological and physical properties in a ridge tilled and a ploughed Luvisol

The present study was conducted to determine the spatial heterogeneity of bulk density, soil moisture, inorganic N, microbial biomass C, and microbial biomass N in the ridge tillage system of Turiel compared to conventional mouldboard ploughing on three sampling dates in May, July, and August. The soil sampling was carried out under vegetation representing the ridge in a high spatial resolution down the soil profile. Bulk density increased with depth and ranged from 1.3 g cm⁻³ at 10 cm depth to 1.6 g cm⁻³ at 35 cm in ploughed plots and from 1.0 g m⁻³ at 5 cm to 1.4 g m⁻³ at 35 cm in the ridges. In the ploughed plots, the contents of microbial biomass C and microbial biomass N remained roughly constant at 215 and 33 μg g⁻¹ soil, respectively, throughout the experimental period. The microbial biomass C/N ratio varied in a small range around 6.4. In the ridged plots, the contents of microbial biomass C and microbial biomass N were 5% and 6% higher compared to the ploughed plots. Highest microbial biomass C contents of roughly 300 μg g⁻¹ soil were always measured in the crowns in July. The lowest contents of microbial biomass C of 85–137 μg g⁻¹ soil were measured in the furrows. The ridges showed strong spatial heterogeneity in bulk density, soil water content, inorganic nitrogen and microbial biomass.     

Soil microbial activity and crop sustainability in a long-term experiment with three soil-tillage and two crop-rotation systems

Reduction in soil disturbance can stimulate soil microbial biomass and improve its metabolic efficiency, resulting in better soil quality, which in turn, can increase crop productivity. In this study we evaluated microbial biomass of C (MB-C) by the fumigation-extraction (FE) or fumigation-incubation (FI) method; microbial biomass of N (MB-N); basal respiration (BR) induced or not with sucrose; metabolic quotient (obtained by the ratio BR/MB-C) induced (qCO₂(S)), or not with sucrose (qCO₂); and crop productivity in a 14-year experiment in the state of Paraná, southern Brazil. The experiment consisted of three soil-tillage systems [no-tillage (NT), conventional tillage (CT) and no-tillage using a field cultivator every 3 years (FC)] and two cropping systems [a soybean–wheat-crop sequence (CS), and a soybean–wheat–white lupin–maize–black oat–radish crop rotation (CR)]. There were six samplings in the 14th year, starting at the end of the winter crop (wheat in the CS and lupin in the CR plots) and finishing at full flowering of the summer crop (soybean in the CS and maize in the CR). Differences in microbiological parameters were greater than those detected in the total C (TCS) and total N (TNS) contents of the soil organic matter (SOM). Major differences were attributed to tillage, and on average NT was higher than the CT in the following parameters: TCS (19%), TNS (21%), MB-C evaluated by FE (74%) and FI (107%), and MB-N (142%). The sensibility of the microbial community and processes to soil disturbance in the tropics was highlighted, as even a moderate soil disturbance every 3 years (FC) affected microbial parameters but not SOM. The BR was the parameter that most promptly responded to soil disturbance, and strong differences were perceived by the ratio of qCO₂ evaluated with samples induced and non-induced with sucrose. At plowing, the qCO₂(S):qCO₂ was five times higher under CT, indicating a C-starving low-effective microbial population in the C-usage. In general, crop rotation had no effect on microbial parameters or SOM. Grain yield was affected by tillage and N was identified as a limiting nutrient. Linear regressions between grain yields and microbial parameters showed that soybean was benefited from improvements in the microbial biomass and metabolic efficiency, but with no significant effects observed for the maize crop. The results also indicate that the turnover of C and N in microbial communities in tropical soils is rapid, reinforcing the need to minimize soil disturbance and to balance inputs of N and C.     

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.     

Microbial mineralization of biochar and wheat straw mixture in soil: A short-term study

A short-term incubation study was carried out to investigate the effect of biochar addition to soil on CO₂ emissions, microbial biomass, soil soluble carbon (C) nitrogen (N) and nitrate–nitrogen (NO₃–N). Four soil treatments were investigated: soil only (control); soil + 5% biochar; soil + 0.5% wheat straw; soil + 5% biochar + 0.5% wheat straw. The biochar used was obtained from hardwood by pyrolysis at 500 °C. Periodic measurements of soil respiration, microbial biomass, soluble organic C, N and NO₃–N were performed throughout the experiment (84 days). Only 2.8% of the added biochar C was respired, whereas 56% of the added wheat straw C was decomposed. Total net CO₂ emitted by soil respiration suggested that wheat straw had no priming effect on biochar C decomposition. Moreover, wheat straw significantly increased microbial C and N and at the same time decreased soluble organic N. On the other hand, biochar did not influence microbial biomass nor soluble organic N. Thus it is possible to conclude that biochar was a very stable C source and could be an efficient, long-term strategy to sequester C in soils. Moreover, the addition of crop residues together with biochar could actively reduce the soil N leaching potential by means of N immobilization.     

Rhizosphere soil microbial index of tree species in a coal mining ecosystem

Microbial characterization of the tree rhizosphere provides important information relating to the screening of tree species for re-vegetation of degraded land. Rhizosphere soil samples collected from a few predominant tree species growing in the coal mining ecosystem of Dhanbad, India, were analyzed for soil organic carbon (SOC), mineralizable N, microbial biomass carbon (MBC), active microbial biomass carbon (AMBC), basal soil respiration (BSR), and soil enzyme activities (dehydrogenase, urease, catalase, phenol oxidase, and peroxidase). Among the tree species studied, Aegle marmelos recorded the highest value for MBC (590 mg kg⁻¹), urease (190.5 μg NH₄⁺-N g⁻¹ h⁻¹), catalase (513 μg H₂O₂ g⁻¹ h⁻¹), dehydrogenase (92.3 μg TPF g⁻¹ h⁻¹), phenol oxidase (0.057 μM g⁻¹ h⁻¹) and BSR/AMBC (0.498 mg CO₂-C mg biomass⁻¹ day⁻¹); Tamarindus indica for mineralizable N (69.5 mg kg⁻¹); Morus alba for catalase (513 μg H₂O₂ g⁻¹ h⁻¹) and phenol oxidase (0.058 μM g⁻¹ h⁻¹); Tectona grandis for peroxidase (0.276 μM g⁻¹ h⁻¹), AMBC/MBC (99.4%), and BSR/MBC (0.108 mg CO₂-C mg biomass⁻¹ day⁻¹); Ficus religiosa for AMBC (128.4 mg kg⁻¹) and BSR (12.85 mg CO₂-C kg⁻¹ day⁻¹); Eugenia jambolana for MBC/SOC (8.03%); Butea monosoperma for AMBC/SOC (1.32%) and Azadirachta indica for BSR/AMBC (0.1134 mg CO₂-C mg biomass⁻¹ day⁻¹). Principal component analysis was employed to derive a rhizosphere soil microbial index (RSMI) and accordingly, dehydrogenase, BSR/MBC, MBC/SOC, EC, phenol oxidase and AMBC were found to be the most critical properties. The observed values for the above properties were converted into a unitless score (0–1.00) and the scores were integrated into RSMI. The tree species could be arranged in decreasing order of the RSMI as: A. marmelos (0.718), A. indica (0.715), Bauhinia bauhinia (0.693), B. monosperma (0.611), E. jambolana (0.601), Moringa oleifera (0.565), Dalbergia sissoo (0.498), T. indica (0.488), Morus alba (0.415), F. religiosa (0.291), Eucalyptus sp. (0.232) and T. grandis (0.181). It was concluded that tree species in coal mining areas had diverse effects on their respective rhizosphere microbial processes, which could directly or indirectly determine the survival and performance of the planted tree species in degraded coal mining areas. Tree species with higher RSMI values could be recommended for re-vegetation of degraded coal mining area.     

Differences in soil CO2 flux between different land use types in mid-subtropical China

It is crucial to advance the understanding of the soil carbon dioxide (CO₂) flux and environmental factors for a better comprehension of carbon dynamics in subtropical ecosystems. Red soil, one of the typical agricultural soils in subtropical China, plays important roles in the global carbon budget due to their large potential to sequester C and replenish atmospheric C through soil CO₂ flux. We examined the relationship between soil CO₂ flux and environmental determinants in four different land use types of subtropical red soil-paddy (P), orchard (O), woodland (W) and upland (U) using static closed chamber method. Objectives were to evaluate the relationship of soil temperature, water-filled pore space (WFPS), and dissolved organic carbon (DOC) with the soil CO₂ flux. Soil CO₂ fluxes were measured on each site about every 14 days between 09:00 and 11:00 a.m. during 14-July 2004 to 25-April 2007 at the experimental station of Heshengqiao at Xianning, Hubei, China. Soil CO₂ fluxes revealed seasonal fluctuations, with the tendency that maximum values occurred in summer, minimum in winter and intermediate values in spring and autumn except for paddy soil when it was submerged. Further, significant differences in soil CO₂ fluxes were observed among the four soils, following the order of P > O > U  W. Average soil CO₂ fluxes were estimated as 901 ± 114, 727 ± 55, 554 ± 22 and 533 ± 27 (±S.D.) g CO₂ m⁻² year⁻¹ in paddy, orchard, upland and woodland soils, respectively. Variations in soil CO₂ flux were related to soil temperature, WFPS, and dissolved organic carbon with a combined R² of 0.49–0.75. Soil temperature was an important variable controlling 26–59% of soil CO₂ flux variability. The interaction of soil temperature and WFPS could explain 31–60% of soil CO₂ flux variations for all the land use types. We conclude that soil CO₂ flux from red soil is under environmental controls, soil temperature being the main variable, which interact with WFPS and DOC to control the supply of readily mineralizable substrates.     

Determinants of annual fluxes of CO2 and N2O in long-term no-tillage and conventional tillage systems in northern France

The greenhouse gases CO₂ and N₂O emissions were quantified in a long-term experiment in northern France, in which no-till (NT) and conventional tillage (CT) had been differentiated during 32 years in plots under a maize–wheat rotation. Continuous CO₂ and periodical N₂O soil emission measurements were performed during two periods: under maize cultivation (April 2003–July 2003) and during the fallow period after wheat harvest (August 2003–March 2004). In order to document the dynamics and importance of these emissions, soil organic C and mineral N, residue decomposition, soil potential for CO₂ emission and climatic data were measured. CO₂ emissions were significantly larger in NT on 53% and in CT on 6% of the days. From April to July 2003 and from November 2003 to March 2004, the cumulated CO₂ emissions did not differ significantly between CT and NT. However, the cumulated CO₂ emissions from August to November 2003 were considerably larger for NT than for CT. Over the entire 331 days of measurement, CT and NT emitted 3160 ± 269 and 4064 ± 138 kg CO₂-C ha⁻¹, respectively. The differences in CO₂ emissions in the two tillage systems resulted from the soil climatic conditions and the amounts and location of crop residues and SOM. A large proportion of the CO₂ emissions in NT over the entire measurement period was probably due to the decomposition of old weathered residues. NT tended to emit more N₂O than CT over the entire measurement period. However differences were statistically significant in only half of the cases due to important variability. N₂O emissions were generally less than 5 g N ha⁻¹ day⁻¹, except for a few dates where emission increased up to 21 g N ha⁻¹ day⁻¹. These N₂O fluxes represented 0.80 ± 0.15 and 1.32 ± 0.52 kg N₂O-N ha⁻¹ year⁻¹ for CT and NT, respectively. Depending on the periods, a large part of the N₂O emissions occurred was probably induced by nitrification, since soil conditions were not favorable for denitrification. Finally, for the period of measurement after 32 years of tillage treatments, the NT system emitted more greenhouses gases (CO₂ and N₂O) to the atmosphere on an annual basis than the CT system.     

Effect of heavy metals contamination on root-derived and organic matter-derived CO2 efflux from soil planted with Zea mays

The CO₂ efflux from loamy Haplic Luvisol and heavy metal (HM) uptake by Zea mays L. were studied under increased HM contamination: Cd, Cu, and Ni up to 20, 1000, and 2500 mg kg⁻¹ soil, respectively. Split-root system with contrasting HM concentrations in both soil halves was used to investigate root-mediated HM translocation in uncontaminated soil zones. To separate root-derived and soil organic matter (SOM)-derived CO₂ efflux from soil, ¹⁴CO₂ pulse labeling of 15-, 25-, and 35-days-old plants was applied. The CO₂ evolution from the bare soil was 10.6 μg C–CO₂ d⁻¹ g⁻¹ (32 kg C–CO₂ d⁻¹ ha⁻¹) and was not affected by HM (except 2500 mg Ni kg⁻¹). The average CO₂ efflux from the soil with maize was about two times higher and amounted for about 22.0 μg C–CO₂ d⁻¹ g⁻¹. Portion of assimilates respired in the rhizosphere decreased with plant development from 6.0 to 7.0% of assimilated C for 25-days-old Zea mays to 0.4–2.0% for 45-days-old maize. The effect of the HM on root-derived ¹⁴CO₂ efflux increased with rising HM content in the following order: Cd < Cu < Ni. In Cu and Ni contaminated soils, shoot and root dry matter decreased to 70% and to 50% of the uncontaminated control, respectively. Plants contained much more HM in the roots than in the shoots. A split-root system with contrasting HM concentrations allowed to trace transport of mobile forms of HM by roots from contaminated soil half into the uncontaminated soil half. The portion of mobile HM forms in the soil (1 M NH₄NO₃ extract) increased with contamination and amounted to 9–16%, 2–6% and 1.5–3.5% for Cd, Cu, and Ni, respectively. Corresponding values for the easily available HM (1 M NH₄OAc extract) were 22–52%, 1–20% and 5–8.5%. Heavy metal availability for plants decreased in the following order: Cd > Cu ≥ Ni. No increase of HM availability in the soil was found after maize cultivation.     

Root activity and carbon metabolism in soils

Summary Two different soils were amended with ¹⁴C-labelled plant material and incubated under controlled laboratory conditions for 2 years. Half the samples were cropped with wheat (Triticum aestivum) 10 times in succession. At flowering, the wheat was harvested and the old roots removed from the soil, so that the soil was continuously occupied by predominantly active root systems. The remaining samples were maintained without plants under the same conditions. During the initial stages of high microbial activity, due to decomposition of the labile compounds, the size of the total microbial biomass was comparable for both treatments, and the metabolic quotient (qCO₂-C = mg CO₂-C·mg^–1 Biomass C·h^–1) was increased by the plants. During the subsequent low-activity decomposition stages, after the labile compounds had been progressively mineralized, the biomass was multiplied by a factor of 2–4 in the presence of plants compared to the bare soils. Nevertheless, qCO₂-C tended to reach similar low values with both treatments. The ¹⁴C-labelled biomass was reduced by the presence of roots and qCO₂-¹⁴C was increased. The significance of these results obtained from a model experiment is discussed in terms of (1) the variation in the substrate originating from the roots and controlled by the plant physiology, (2) nutrient availability for plants and microorganisms, (3) soil biotic capacities and (4) increased microbial turnover rates induced by the roots.     

Soil carbon sequestration with continuous no-till management of grain cropping systems in the Virginia coastal plain

Carbon sequestration in agroecosystems represents a significant opportunity to offset a portion of anthropogenic CO₂ emissions. Climatic conditions in the Virginia coastal plain and modern production practices make it possible for high annual photosynthetic CO₂ fixation. There is potential to sequester a substantial amount of C, and concomitantly improve soil quality, with the elimination of tillage for crop production in this region. The objectives of our research were to: (1) measure C sequestration rate with continuous no-till management of grain cropping systems of the Virginia middle coastal plain; (2) determine the influence of biosolids application history on C content and its interaction with tillage management; and (3) evaluate the impact of continuous no-till C stratification as an indicator of soil quality. Samples were collected from 63 sites in production fields using a rotation of corn (Zea mays L.)–wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.)/soybean double-crop (Glysine max L.) across three soil series [Bojac (coarse-loamy, mixed, semiactive, thermic Typic Hapludults), Altavista (fine-loamy, mixed semiactive, thermic Aquic Hapludults), and Kempsville (fine-loamy, siliceous, subactive, thermic Typic Hapludults)] with a history of continuous no-till management ranging from 0 to 14 years. Thirty-two of the sites had a history of biosolids application. Five soil cores were collected at each site from 0–2.5, 2.5–7.5 and 7.5–15 cm and analyzed for bulk density and soil C. Bulk density in the 0–2.5 cm layer decreased and C stratification ratio (0–2.5 cm:7.5–15 cm) increased with increasing duration of continuous no-till due to the accumulation of organic matter at the soil surface. A history of biosolids application resulted in an increase of 4.19 ± 1.93 Mg C ha⁻¹ (0–15 cm). Continuous no-till resulted in the sequestration of 0.308 ± 0.280 Mg C ha⁻¹ yr⁻¹ (0–15 cm). Our results provide quantitative validation of the C sequestration rate and improved soil quality with continuous no-till management in the region using on-farm observations.     

Soil aggregation and distribution of carbon and nitrogen in different fractions under long-term application of compost in rice–wheat system

Soil organic matter improves the physical, chemical and biological properties of soil, and crop residue recycling is an important factor influencing soil organic matter levels. We studied the impact of continuous application of rice straw compost either alone or in conjunction with inorganic fertilizers on aggregate stability and distribution of carbon (C) and nitrogen (N) in different aggregate fractions after 10 cycles of rice–wheat cropping on a sandy loam soil at Punjab Agricultural University research farm, Ludhiana, India. Changes in water stable aggregates (WSA), mean weight diameter (MWD), aggregate-associated C and N, total soil C and N, relative to control and inorganically fertilized soil were measured. Total WSA were significantly (p = 0.05) higher for soils when rice straw compost either alone or in combination with inorganic fertilizers was applied as compared to control. The application of rice straw compost either alone or in combination with inorganic fertilizers increased the macroaggregate size fractions except for 0.25–0.50 mm fraction. The MWD was significantly (p = 0.05) higher in plots receiving rice straw compost either alone at 8 tonnes ha⁻¹ (0.51 mm at wheat harvest and 0.41 mm at rice harvest) or at 2 tonnes ha⁻¹ in combination with inorganic fertilizers (0.43 and 0.38 mm) as compared to control (0.34 and 0.33 mm) or inorganically fertilized plots (0.33 and 0.31 mm). The macroaggregates had higher C and N density compared to microaggregates. Application of rice straw compost at 2 tonnes ha⁻¹ along with inorganic fertilizers (IN + 2RSC) increased C and N concentration significantly over control. The C and N concentration increased further when rice straw compost at 8 tonnes ha⁻¹ (8RSC) was added. It is concluded that soils can be rehabilitated and can sustain the soil C and N levels with the continuous application of rice straw compost either alone or in combination with inorganic fertilizers. This will also help in controlling the rising levels of atmospheric carbon dioxide.     

Carbon sequestration and relationship between carbon addition and storage under rainfed soybean–wheat rotation in a sandy loam soil of the Indian Himalayas

Soil organic matter (SOM) contributes to the productivity and physical properties of soils. Although crop productivity is sustained mainly through the application of organic manure in the Indian Himalayas, no information is available on the effects of long-term manure addition along with mineral fertilizers on C sequestration and the contribution of total C input towards soil organic C (SOC) storage. We analyzed results of a long-term experiment, initiated in 1973 on a sandy loam soil under rainfed conditions to determine the influence of different combinations of NPK fertilizer and fertilizer + farmyard manure (FYM) at 10 Mg ha⁻¹ on SOC content and its changes in the 0–45 cm soil depth. Concentration of SOC increased 40 and 70% in the NPK + FYM-treated plots as compared to NPK (43.1 Mg C ha⁻¹) and unfertilized control plots (35.5 Mg C ha⁻¹), respectively. Average annual contribution of C input from soybean (Glycine max (L.) Merr.) was 29% and that from wheat (Triticum aestivum L. Emend. Flori and Paol) was 24% of the harvestable above-ground biomass yield. Annual gross C input and annual rate of total SOC enrichment were 4852 and 900 kg C ha⁻¹, respectively, for the plots under NPK + FYM. It was estimated that 19% of the gross C input contributed towards the increase in SOC content. C loss from native SOM during 30 years averaged 61 kg C ha⁻¹ yr⁻¹. The estimated quantity of biomass C required to maintain equilibrium SOM content was 321 kg ha⁻¹ yr⁻¹. The total annual C input by the soybean–wheat rotation in the plots under unfertilized control was 890 kg ha⁻¹ yr⁻¹. Thus, increase in SOC concentration under long-term (30 years) rainfed soybean–wheat cropping was due to the fact that annual C input by the system was higher than the required amount to maintaining equilibrium SOM content.