Assessing the effect of crop residue removal on soil organic carbon storage and microbial activity in a no‐till cropping system

Changes in agricultural management strategies have received much attention in recent years with a view to increasing or maintaining the amount of carbon (C) sequestered as soil organic C (SOC). In many parts of the world, minimum or no‐till management has been promoted as a means of improving soil quality, reducing losses of erosion and potentially increasing SOC stocks. However, no‐till systems can become problematic and potentially disease‐prone, especially due to high crop residue loadings. Consequently, residue removal either by harvesting or burning off may be employed to reduce these pressures. Here, we examined the effect of crop residue removal on C storage in soil that had been under no‐till management for 20 yr. We predicted improved physical properties (i.e. lower bulk density) and greater microbial activity under the residue retention soils due to greater readily available C and nutrients derived from crop residues. In contrast, we predicted relative reductions in SOC in the no residue soils due to a lack of available residue‐derived C for microbial use. Residue removal caused a relative C loss from the soil, which was related to C input, amount of nutrient availability and microbial activity. We demonstrate the importance of maintaining crop residue cover in no‐till cropping systems for soil function and highlight the potentially deleterious effects of changing management strategy to increased residue harvesting or removal by burning.     

Effects of reduced tillage,crop residue management and manure application practices on crop yields and soil carbon sequestration on an Andisol in northern Japan

Abstract

Soil carbon sequestration in agricultural lands has been deemed a sustainable option to mitigate rising atmospheric CO₂ levels. In this context, the effects of different tillage and C input management (residue management and manure application) practices on crop yields, residue C and annual changes in total soil organic C (SOC) (0–30 cm depth) were investigated over one cycle of a 4-year crop rotation (2003–2006) on a cropped Andisol in northern Japan. For tillage practices, the effects of reduced tillage (no deep plowing, a single shallow harrowing for seedbed preparation [RT]) and conventional deep moldboard plow tillage (CT) were compared. The combination of RT, residue return and manure application (20 Mg ha^?1 in each year) increased spring wheat and potato yields significantly; however, soybean and sugar beet yields were not influenced by tillage practices. For all crops studied, manure application enhanced the production of above-ground residue C. Thus, manure application served not only as a direct input of C to the soil, but the greater crop biomass production engendered enhanced subsequent C inputs to the soil from residues. The SOC contents in both the 0–5 cm and 5–10 cm layers of the soil profile were greater under RT than under CT treatments because the crop residue and manure were densely incorporated into the shallow soil layers. Comparatively, neither tillage nor C input management practices had significant effects on annual changes in SOC content in either the 10–20 cm or 20–30 cm layers of the soil profile. When soil C sequestration rates, as represented by annual changes in total SOC (0–30 cm), were assessed on a total soil mass basis, an anova showed that tillage practices had no significant effect on total C sequestration, but C input management practices had significant positive effects (P ≤ 0.05). These results indicate that continuous C input to the soil through crop residue return and manure application is a crucial practice for enhancing crop yields and soil C sequestration in the Andisol region of northern Japan.     

Natural 13C abundance and soil carbon dynamics under long‐term residue retention in a no‐till maize system

Residue retention and reduced tillage are both conservation agricultural practices that may enhance soil organic carbon (SOC) stabilization in soil. We evaluated the long‐term effects of no‐till (NT) and stover retention from maize on SOC dynamics in a Rayne silt loam Typic Hapludults in Ohio. The six treatments consisted of retaining 0, 25, 50, 75, 100 and 200% of maize residues on each 3 × 3 m plot from the crop of previous year. Soil samples were obtained after 9 yrs of establishing the experiment. The whole soil (0–10 and 10–20 cm of soil depths) samples under different treatments were analysed for total C, total N, recalcitrant C (NaOCl treated sample) and ¹³C isotopic abundance (0–10 cm soil depth). Complete removal of stover for a period of 9 yrs significantly (P < 0.01) decreased soil C content (15.5 g/kg), whereas 200% of stover retention had the maximum soil C concentration (23.1 g/kg). Relative distribution of C for all the treatments in different fractions comprised of 55–58% as labile and 42–45% as recalcitrant. Retention of residue did not significantly affect total C and N concentration in 10–20 cm depth. ¹³C isotopic signature data indicated that C₄‐C (maize‐derived C) was the dominant fraction of C in the top 0–10 cm of soil layer under NT with maize‐derived C accounting for as high as 80% of the total SOC concentration. Contribution of C₄‐C or maize‐derived C was 71–84% in recalcitrant fraction in different residue retained plots. Residue management is imperative to increase SOC concentrations and long‐term agro‐ecosystem necessitates residue retention for stabilizing C in light‐textured soils.     

Differences in CO2 and N2O emission rates following crop residue incorporation with or without field burning: A case study of adzuki bean residue and wheat straw

In the context of sustainable soil-quality management and mitigating global warming, the impacts of incorporating raw or field-burned adzuki bean (Vigna angularis (Willd.) Ohwi & Ohashi) and wheat (Triticum aestivum L.) straw residues on carbon dioxide (CO₂) and nitrous oxide (N₂O) emission rates from soil were assessed in an Andosol field in northern Japan. Losses of carbon (C) and nitrogen (N) in residue biomass during field burning were much greater from adzuki bean residue (98.6% of C and 98.1% of N) than from wheat straw (85.3% and 75.3%, respectively). Although we noted considerable inputs of carbon (499 ± 119 kg C ha^–1) and nitrogen (5.97 ± 0.76 kg N ha^–1) from burned wheat straw into the soil, neither CO₂ nor N₂O emission rates from soil (over 210 d) increased significantly after the incorporation of field-burned wheat straw. Thus, the field-burned wheat straw contained organic carbon fractions that were more resistant to decomposition in soil in comparison with the unburned wheat straw. Our results and previously reported rates of CO₂, methane (CH₄) and N₂O emission during wheat straw burning showed that CO₂-equivalent greenhouse gas emissions under raw residue incorporation were similar to or slightly higher than those under burned residue incorporation when emission rates were assessed during residue burning and after subsequent soil incorporation.     

An evaluation of C and N on fresh and aged crop residue from mixed long-term no-till cropping systems

Conservation crop residue management increases soil organic carbon (SOC) storage, nutrient cycling and availability and improves soil quality. This study was conducted to evaluate the amount of residue biomass, residue carbon to nitrogen (C:N) ratio, residue carbon (C) and nitrogen (N), and residue N fertilizer deficit (supplemental N fertilizer requirement) from crop residue decomposition in long-term no-till production. Aboveground aged and fresh residues were collected in spring 2011 and fall 2012, respectively. Results showed slightly greater residue dry matter weight in aged residue than fresh residue. C:N ratios were wider in fresh residue than the aged residue. Both aged and fresh residue also showed wider C:N ratio in the corn (Zea mays L.)-soybean (Glycine max L.) rotation (66.6 and 64.4, respectively) and narrower C:N ratio in the spring wheat (Triticum aestivum L.)-winter wheat (Triticum aestivum L.)-alfalfa (Medicago sativa L.)-alfalfa-corn (Zea mays L.)-soybean (Glycine max L.) (45.6 and 35.7, respectively). Individual fresh crop residues showed narrower C:N ratios for legume and cover crops than non-legume crops. Analysis of potential supplemental N fertilizer requirements showed greater potential N requirement for the fresh residue than the aged residue.     

Distribution of charred plant fragments in particle size fractions of Japanese volcanic ash soils

Abstract

To gain a better understanding of the distribution of charred plant fragment C (CPFC) and its contribution to organic C (OC) in the particle size fractions of Japanese volcanic ash soils, each of four soil samples was divided into six particle size fractions, namely three sand-sized aggregate (20–53, 53–212 and 212–2,000 µm) fractions, one silt-sized aggregate (2–20 µm) fraction, and two clay-sized aggregate (< 0.2 and 0.2–2 µm) fractions. Furthermore, after HCl–HF treatment of these aggregate fractions, sub-fractions of less than specific gravity (s.g.) 1.6 g cm^?3 (< 1.6 fraction) were isolated using s.g. 1.6 g cm^?3 sodium polytungstate solution. Microscopic observation indicated that the charred plant fragments, which are black or blackish brown, were the main components in the < 1.6 fractions. Therefore, the OC in this fraction was designated as CPFC. In all the soils studied, the quantitative distribution of the CPFC of the silt-sized aggregate fractions to total CPFC of whole soils, ranging from 59 to 84%, was greatest among the aggregate fractions. The sum of the distribution (%) values of the CPFC in the three sand-sized aggregate fractions varied from 6.9 to 33%, while that in the two clay-sized aggregate fractions ranged from 1.1 to 9.4%. Similar to the CPFC, in all soils, the quantitative distribution of the OC in the aggregate fractions was greater in the silt-sized aggregate fractions (52–76%) than in the other aggregate fractions (0.1–20%). In all soils, the quantitative contribution of total CPFC to total OC of whole soils ranged from 10 to 28%. The CPFC/OC values in the aggregate fractions were 21% or more in 10 samples from a total of 24 fractions, with a maximum value of 34%. On the basis of the findings obtained in the present study, it is assumed that in Japanese volcanic ash soils the silt-sized fraction is an important reservoir of CPFC and OC, and CPFC merits attention as one of the constituents of OC in particle size fractions.     

Decomposition of mulched versus incorporated crop residues: Modelling with PASTIS clarifies interactions between residue quality and location

Crop residue management has been shown to significantly affect the decomposition process of plant debris in soil. In previous studies examining this influence, the extrapolation of laboratory data of carbon and/or nitrogen mineralization to field conditions was often limited by a number of interactions that could not be taken into account by a mere experimental approach. Therefore, we demonstrated the interactive effect between crop residue location in soil (mulch vs. incorporation) and its biochemical and physical quality, in repacked soil columns under artificial rain. Decomposition of ¹³C and ¹⁵N labelled rape and rye residues, with associated C and N fluxes, was analysed using the mechanistic model PASTIS, which turned out to be necessary to understand the interacting factors on the C and N fluxes. The influence of soil and residue water content on decomposition and nitrification was evaluated by the moisture limitation factor of PASTIS. This factor strongly depended on residue location and to a smaller extent on physical residue properties, resulting in a lower decomposition rate of about 35% for surface placed compared to incorporated residues. Irrespective of its placement, the biochemical residue quality (e.g. N availability for decomposition, amount of soluble compounds and lignin) was responsible for a faster and more advanced decomposition of about 15% in favour of rye compared to rape, suggesting only a limited interaction between residue quality and its location. Net N mineralization after nine weeks was larger for rye than for rape, equivalent to 59 and 10 kg NO₃⁻-N ha⁻¹ with incorporation, and 71 and 34 kg NO₃⁻-N ha⁻¹ with mulch, respectively. This net N mineralization in soil resulted from the interaction between soil water content, depending on residue placement, and N availability, which was determined by the biochemical residue quality. Moisture limitation appeared more important than N limitation in the decomposition of mulched residues. Modelling of gross N mineralization and immobilization also revealed that leaving crop residues at the soil surface may increase the risk of nitrate leaching compared to residue incorporation, if (i) soil water content under mulch is larger than with residue incorporation (more gross N mineralization), and (ii) availability to the applied C-source is limited (less gross N immobilization). Scenario analyses with PASTIS confirmed the importance of moisture conditions on the decomposition of mulched residues and the small interaction between biochemical crop residue quality and its location in soil.     

Effects of crop residue incorporation and inorganic potassium fertilization on soil potassium supply power

Crop residues can be used as an effective replacement for inorganic potassium (K) fertilizers. However, the impact of the type and quantity of crop residue on soil K supply capacity in different soil types remains poorly known. To investigate the effects of crop residue incorporation rate and inorganic K fertilization on apparent K balance and soil K supply capacity in cotton-wheat cropping system, we conducted two, six-year field experiments on two soil types in Nanjing and Dafeng, China. At both sites, crop residue incorporation ameliorated the negative K balance and improved the soil water-soluble, exchangeable, and non-exchangeable K (WSK, EK, and NEK) contents. The soil WSK, EK and NEK contents were positively correlated with the net K accumulation at both sites across two soil layers. However, the correlations between net K accumulation and quantity-intensity related parameters varied with the soil types. Treatment with wheat straw (9000 kg ha^?1) + cotton residue (7500 kg ha^?1) at both sites performed better than the 300 kg K₂O ha^?1 treatment in terms of balancing soil K depletion and keeping adequate K fertility status. Incorporation of adequate crop residues performed similarly to inorganic potash in improving the soil K supply power.     

Contributions of incorporated residue and living roots to aggregate-associated and microbial carbon in two soils with different clay mineralogy

We conducted a study to investigate the role of aggregates in the stabilization of residue‐ and root‐derived C in an illitic Mollisol and a kaolinitic Oxisol under the following treatments: (i) incorporated residue, (ii) growing plants, and (iii) both incorporated residue and growing plants. Residue‐C dynamics were followed in soils incubated with ¹³C‐labelled wheat residue with and without unlabelled growing wheat plants. Root‐C was traced by growing wheat plants with and without unlabelled wheat residue in a ¹³CO₂‐labelling chamber. After 46 and 76 incubation days, residue‐ and root‐C were measured in four aggregate size classes and in microbial‐C. Both soils had greater residue‐derived than root‐derived total aggregate‐associated C at day 76, which we attributed to the larger residue‐C than root‐C inputs at the start of the experiment. On an aggregate basis, the ratio of residue‐derived over root‐derived C decreased in most size fractions over time, indicating a greater potential for longer‐term root‐C than residue‐C stabilization by aggregates in both soils. At both sampling days, all aggregates > 53 µm had greater residue‐C concentrations in the illitic soil than in the kaolinitic soil and this difference increased with increasing aggregate size. This suggested a greater affinity of illite clay than kaolinite clay to bind with fresh residue‐derived compounds into larger aggregates and hence a greater importance of aggregates in stabilizing residue‐C in illitic compared with kaolinitic soils. The stabilization of root‐C by aggregates was less affected by clay mineralogy and thus less dependent on the affinity of clay minerals to bind with root‐derived compounds.     

Maize grain production,plant nutrient concentration and soil chemical properties in response to different residue levels from two previous crops

ABSTRACT

The incorporation of previous crop residues in agricultural management benefits soil fertility, crop production, and environment. However, there is no enough information about maximum residue application level without negative effect over next crop yield. To evaluate maize (Zea mays L.) yield under short-time conservation management with incorporation and/or importation of different residue levels, a biannual rotation experiment was conducted in ash volcanic soil in south-central Chile. The experiment consisted of two previous crops, canola (Brassica napus L.) and bean (Phaseolus vulgaris L.), and four levels of residue incorporation (0%, 50%, 100%, and 200% of generated residue; from 0 to 21.4?Mg?ha^?1 for canola and from 0 to 19.0?Mg?ha^?1 for bean). Previous crop species and residue level affected some nutrients concentrations in grain and plant and some soil chemical properties, without effect in maize yield, which averaged 16.6?Mg?ha^?1. Bean residue increased Ca and reduced S in maize plant, increasing soil P, Ca, Mg and K (P?<?0.05). Maize grain Ca content was positively and proportionally affected by canola residue level and negatively and proportionally affected by bean residue level. All canola residue levels increased soil pH and Mg, but the highest level reduced soil S; soil P concentration increased proportionally with bean residue level. The highest bean residue level increased soil S. Different crop and levels of residue did not affect maize yield but did some plant nutrient concentration, and also affected some soil chemical properties.     

Changes in P fractions and sorption in an Alfisol following crop residues application

With the emphasis on sustainable agriculture, attention has been increasingly turning to recycling of crop residues as a component of fertility management strategies for tropical soils. We assessed the effects of soybean residue (SR) and wheat residue (WR) applied either alone or in combination with fertilizer P (FP) on dynamics of labile P, distribution of P fractions, and P sorption in a semiarid tropical Alfisol by conducting a 16 w long incubation experiment. The amount of P added through crop residues, FP or their combinations was kept constant at 10 mg P (kg soil)^–1. Addition of SR or WR resulted in net increase of labile inorganic (P_i) and organic P (P_o) and microbial P throughout the incubation period, except that the WR decreased labile P_i during first 2 w due to P_i immobilization. The P immobilization associated with WR addition was, however, offset when fertilizer P was combined with WR. Generally, the increases in labile‐P fractions were larger with the SR and SR+FP than with the WR and WR+FP. The sequential fractionation of soil P at the end of 16 w indicated that a major part of added fertilizer P transformed into moderately labile and stable P fractions as evident from the increased NaOH‐P_i and HCl‐P in the FP treatment. In contrast, the addition of SR and WR alone or in combination with FP favored a build‐up in NaHCO₃‐P_i and ‐P_o and NaOH‐P_o fractions while causing a decrease in NaOH‐P_i and HCl‐P fractions. The addition of these crop residues also effectively decreased the P‐sorption capacity and hence reduced the standard P requirement of the soil (i.e., the amount of P required to maintain optimum solution P concentration of 0.2 mg P l^–1) by 24%–43%. Results of the study, thus, imply that soybean and wheat crop residues have the potential to improve P fertility of Alfisols by decreasing P‐sorption capacity and by redistributing soil P in favor of labile‐P fractions and promoting accretion of organic P.     

Earthworm populations and growth rates related to long-term crop residue and tillage management

Conventional tillage creates soil physical conditions that may restrict earthworm movement and accelerate crop residue decomposition, thus reducing the food supply for earthworms. These negative impacts may be alleviated by retaining crop residues in agroecosystems. The objective of this study was to determine the effects of various tillage and crop residue management practices on earthworm populations in the field and earthworm growth under controlled conditions. Population assessments were conducted at two long-term (15+ years) experimental sites in Québec, Canada with three tillage systems: moldboard plow/disk harrow (CT), chisel plow or disk harrow (RT) and no tillage (NT), as well as two levels of crop residue inputs (high and low). Earthworm growth was assessed in intact soil cores from both sites. In the field, earthworm populations and biomass were greater with long-term NT than CT and RT practices, but not affected by crop residue management. Laboratory growth rates of Aporrectodea turgida (Eisen) in intact soil cores were affected by tillage and residue inputs, and were positively correlated with the soil organic C pool, suggesting that tillage and residue management practices that increase the soil organic C pool provide more organic substrates for earthworm growth. The highest earthworm growth rates were in soils from RT plots with high residue input, which differed from the response of earthworm populations to tillage and residue management treatments in the field. Our results suggest that tillage-induced disturbance probably has a greater impact than food availability on earthworm populations in cool, humid agroecosystems.     

Temporal variations of crop residue effects on soil N transformation depend on soil properties as well as residue qualities

Purple soils (Eutric Regosols) are widely distributed in humid subtropical Southwest China. They are characterized by high nitrification activities, with risks of severe NO₃^? leaching. Incorporation of crop residues is considered an effective method to reduce NO₃^? loss. In the present study, we compared the effects of alfalfa, rice straw, and sugarcane bagasse on gross N transformation turnover in a purple soil (purple soil, pH 7.62) compared with those in an acid soil (acid soil, pH 5.26), at 12 h, 3 months, and 6 months after residue incorporation. The gross N transformation rates were determined by ¹⁵N tracing. All tested crop residues stimulated the gross N mineralization rates, but reduced the net mineralization rates in both soils at 12 h after residue incorporation; however, the extent of the effect varied with the crop residue qualities, with rice straw having the strongest effects. Crop residues reduced net nitrification rates by depressing gross autotrophic nitrification rates and stimulating NO₃^? immobilization rates in the purple soil, particularly after rice straw incorporation (net nitrification rate decreased from 16.72 mg N kg^?1 d^?1 in the control to ??29.42 mg N kg^?1 d^?1 at 12 h of residue incorporation); however, crop residues did not affect the gross autotrophic nitrification rates in the acid soil. Crop residue effects subsided almost completely within 6 months, with sugarcane bagasse showing the longest lasting effects. The results indicated that crop residues affected the N transformation rates in a temporal manner, dependent on soil properties and residue qualities.     

Transformations and recovery of residue and fertilizer nitrogen-15 in a sandy Lixisol of West Africa

 The fate of ¹⁵N-labeled plant residues from different cover-cropping systems and labeled inorganic N fertilizer in the organic, soil mineral, microbial biomass and soil organic matter (SOM) particle-size fractions was investigated in a sandy Lixisol. Plant residues were from mucuna (legume), lablab (legume), imperata (grass), maize (cereal) and mixtures of mucuna or lablab with imperata or maize, applied as a surface mulch. Inorganic N fertilizer was applied as ¹⁵N-(NH₄)₂SO₄ at two rates (21 and 42 mg N kg^–1 soil). Total N release from mucuna or lablab residues was 2–3 times higher than from the other residues, whereas imperata immobilized N throughout the study period. In contrast, ¹⁵N was mineralized from all the plant residues irrespective of the mineralization–immobilization pattern observed for total N. After 168 days, 69% of soil mineral N in mucuna- or lablab-mulched soils was derived from the added residues, representing 4–8% of residue N, whereas 9–30% of inorganic N was derived from imperata, maize and the mixed residues. At the end of the study, 4–19% of microbial biomass N was derived from the added residue/fertilizer-N, accounting for 1–3% of added residue-N. Averaged across treatments, particulate SOM fractions accounted for less than 1% of the total soil by weight but contained 20% of total soil C and 8% of soil N. Soils amended with mucuna or lablab incorporated more N in the 250–2000 μm SOM pool, whereas soil amended with imperata or the mixed residues incorporated similar proportions of labeled N in the 250–2000 μm and 53–250 μm fractions. In contrast, in soils receiving the maize or inorganic fertilizer-N treatments, higher proportions of labeled N were incorporated into the 53–250 μm than the 250–2000 μm fractions. The relationship between these differences in residue/fertilizer-N partitioning into different SOM particle-size fractions and soil productivity is discussed. Received: 12 March 1999     

Impacts of organic residue management on the soil C dynamics in a tropical eucalypt plantation on a nutrient-poor sandy soil after three rotations

Harvest residue management is a key issue for the sustainability of short rotation plantations of fast growing tree species established on poor soils and for potential carbon (C) accretion in many soils. By measuring the C isotope composition (δ¹³C) of different soil organic matter fractions and microbial respiration, we examined the effects of organic residue management at harvest on soil C dynamics in a tropical eucalypt plantation established on a savannah at the end of the third rotation on three treatments repeated at each harvest. We compared plots where the standard harvesting method in Congolese commercial plantations was applied (SWH; only removing the debarked commercial-sized boles) with plots where all the aboveground biomass and the litter layer from the previous rotations were removed at harvest (R) and plots where the residues from a whole tree harvest treatment were added (double slash, DS). Organic residue removal decreased the accretion of eucalypt-derived C in coarse and fine particulate organic matter (POM) fractions and in the organo-mineral fraction, resulting in 44% less total soil C in the top soil (0–0.05 m) but did not affect the amount of savannah-derived C in any SOM fraction. In contrast, increasing the amount of harvest residue by 36% in comparison with the reference practice did not increase the amount of C stored in the soil nor the accretion of eucalypt-derived C in any soil organic matter fractions, but the amount of savannah-derived C remaining in the coarse POM fraction was higher. We concluded that carbon accretion may be limited by the low C saturation level of these sandy-structured soils but that higher rates of residue retention may reduce priming on older savannah-derived C.     

Rice residue incorporation and nitrogen application: effects on yield and micronutrient transformations under rice–wheat cropping system

Abstract

Despite being a major domain of global food supply, rice–wheat (RW) cropping system is questioned for its contribution to biomass burning in Indo-Gangetic Plains (IGP). Enhancing the yield and soil quality properties in this system is therefore necessary to reduce environmental degradation and maintain agricultural productivity. A field experiment evaluated the effects of soil management practices such as rice residue (RS) incorporation, and nitrogen (N) application on crop yield and micronutrients transformations in a RW cropping system of north-western India. The results revealed that N application (120?kg N ha^?1) and RS incorporation (7.5 t ha^?1) significantly increased micronutrients cations and crop yield compared with no-residue (RS₀). Irrespective of N application, crop grain yield under RS incorporation (Rs_7.5 t ha^?1) was significantly higher than RS₀ incorporation. Significant increase in all the micronutrient transformations was recorded in N₁₂₀/Rs_7.5 t ha^?1 compared with RS₀. Among different fractions, crystalline Fe bound in Zn, Mn, and Cu and amorphous Fe oxide in Fe fractions were the dominant fractions under N application (N₁₂₀) and RS incorporation (RS_7.5) treatment. Our study showed that application of N₁₂₀ followed by RS_7.5 can be more sustainable practice under RW cropping system for improvement in micronutrients availability and crop yield. This practice also provides an opportunity to incorporation of crop residues as an alternative to burning, which causes severe air pollution in the RW cropping system in the IGP.     

Influence of repeated prescribed burning on incorporation of C from cellulose by forest soil fungi as determined by RNA stable isotope probing

Repeated prescribed burning is frequently used as a forest management tool and can influence soil microbial diversity and activity. Soil fungi play key roles in carbon and nutrient cycling processes and soil fungal community structure has been shown to alter with increasing burning frequency. Such changes are accompanied by changes to soil carbon and nitrogen pools, however, we know little regarding how repeated prescribed burning alters functional diversity in soil fungal communities. We amended soil with ¹³C-cellulose and used RNA stable isotope probing to investigate the effect of biennial repeated prescribed burning over a 34-year period on cellulolytic soil fungi. Results indicated that repeated burning altered fungal community structure. Moreover, fungal community structure and diversity in ¹²C and ¹³C fractions from the unburned soil were not significantly different from each other, while those from the biennial burned soils differed from each other. The data indicate that fewer active fungi in the biennially burned soil incorporated ¹³C from the labelled cellulose and that repeated prescribed burning had a significant impact on the diversity of an important functional group of soil fungi (cellulolytic fungi) that are key drivers of forest soil decomposition and carbon cycling processes.     

Contribution of crop residue carbon to soil respiration at a northern Prairie site using stable isotope flux measurements

Heterotrophic respiration from agricultural soils can be differentiated as originating from microbial decomposition of recent litter inputs or crop residue carbon (CRC) and resident soil organic carbon (SOC) pools of varying age and stages of decomposition. Our objective was to determine the relative contributions of these pools to respiration in a northern agroecosystem where the non-growing season is long. A tunable diode laser trace gas analyzer was used to determine atmospheric stable C isotope ratio (δ¹³C) values and ¹²CO₂ and ¹³CO₂ fluxes over an agricultural field in the Red River Valley of southern Manitoba, Canada. Measurement campaigns were conducted in the fall of 2006 and spring of 2007 following harvest of a maize (C₄) crop from soil having SOC derived from previous C₃ crops. Stable CO₂ isotopologue gradients were measured from the center of four 200 × 200 m experimental plots, and fluxes were calculated using the aerodynamic flux gradient method. The soil in two of the experimental plots underwent intensive tillage, while the other two plots were managed using a form of reduced tillage. Approximately 70% and 20-30% of the total respiration flux originated from the maize C₄-CRC during the fall of 2006 and spring of 2007, respectively. At least 25% of the maize residue was lost to respiration during this non-growing period. No difference in the partitioning of heterotrophic respiration into that derived from CRC and SOC was detected between the intensive tillage and recently established reduced tillage treatments at the site.     

An in-situ Technique for Producing Low-Cost Agricultural Biochar

Application of biochar to agricultural soils is effective to sequester atmospheric carbon and improve soil quality, but current pyrolysis and transportation costs are high, making biochar too costly to be used at the field scale. This study developed a new in-situ technique, burning and soil covering(B-SC), which can be used by farmers for production of biochar with crop residue. In this study,the air-dried feedstocks, elephant grass and corn residue, were burnt in situ for biochar production in the field. After approximately 90% of the leaves were combusted, the burning process was dramatically slowed down by covering the feedstock with soil. The biochar yield averaged 18.0 ± 1.3(n = 15) and 13.7 ± 1.3(n = 10) kg per 100 kg air-dried feedstock for the elephant grass and corn residue,respectively. The biochar properties were suitable for soil improvement. The inputs for biochar production of the B-SC process only included low labor force, open field, feedstock(e.g., grass and crop residue), and simple tools. The operation time for processing 10 kg of the corn residue by an individual farmer was 24.4 ± 4.1 min(n = 10). As compared with the conventional field burning process, the B-SC process drastically shortened the time for biomass burning and generated a significantly lower emission of smoke and thermal energy. This simple technique can be particularly practical and effective for farmers to improve the soils of poor quality in China.     

Differential and successive effects of residue quality and soil mineral N on water-stable aggregation during crop residue decomposition

Residue quality has been shown to influence soil water-stable aggregation (WSA) during crop residue decomposition, but there is still little information about its interactive effect with soil mineral N availability. The aim of this study was to determine the effect of soil mineral N on WSA during the decomposition of two high-C/N crop residues (wheat straw with C/N = 125.6 and miscanthus straw with C/N = 311.3). The two crop residues were combined with three mineral N addition rates (0, 60, and 120 mg N kg⁻¹ dry soil). Respiration, soil mineral N content, and WSA (expressed as mean-weight diameter, MWD) were measured on several dates during a 56-d incubation. The effect of decomposing crop residues on WSA followed two phases. (i) Between 0 and 7 d, the increase in WSA was related to intrinsic residue quality with higher decomposability of the wheat straw resulting in higher WSA. (ii) Thereafter, and until the end of the experiment, mineral N addition rates had a predominant but negative influence on WSA. In this second phase, the average MWD of residue-treated soils was 0.92, 0.55, and 0.44 mm for the 0, 60 and 120 mg N kg⁻¹ dry soil addition rates, respectively. Mineral N addition which did result in higher crop residue decomposition did not lead to higher WSA. WSA during crop residue decomposition is therefore not simply positively related to the induced microbial activity, and changes in microbial community composition with differential effects on WSA must be involved. The impact of high-C/N crop residues inputs on WSA, initially assumed to be low, could actually be strong and long-lasting in situations with low soil mineral N content.