A re‐evaluation of the enriched labile soil organic matter fraction

Identifying ‘functional' pools of soil organic matter and understanding their response to tillage remains elusive. We have studied the effect of tillage on the enriched labile fraction, thought to derive from microbes and having an intermediate turnover time. Four soils, each under three regimes, long‐term arable use without tillage (NT), long‐term arable under conventional tillage (CT), and native vegetation (NV), were separated into four aggregate size classes. Particle size fractions of macro‐ (250–2000 μm) and microaggregates (53–250 μm) were isolated by sonication and sieving. Subsequently, densiometric and chemical analyses were made on fine‐silt‐sized (2–20 μm) particles to isolate and identify the enriched labile fraction. Across soils, the amounts of C and N in the particle size fractions were highly variable and were strongly influenced by mineralogy, specifically by the contents of Fe and Al oxides. This evidence indicates that the fractionation procedure cannot be standardized across soils. In one soil, C associated with fine‐silt‐sized particles derived from macroaggregates was 567 g C m^?2 under NV, 541 g C m^?2 under NT, and 135 g C m^?2 under CT, whereas C associated with fine‐silt‐sized particles derived from microaggregates was 552, 1018, 1302 g C m^?2 in NV, NT and CT, respectively. These and other data indicate that carbon associated with fine‐silt‐sized particles is not significantly affected by tillage. Its location is simply shifted from macroaggregates to microaggregates with increasing tillage intensity. Natural abundance ¹³C analyses indicated that the enriched labile fraction was the oldest fraction isolated from both macro‐ and microaggregates. We conclude that the enriched labile fraction is a ‘passive' pool of soil organic matter in the soil and is not derived from microbes nor sensitive to cultivation.     

Changes in soil organic carbon and its chemical fractions under different tillage practices on loess soils of the Guanzhong Plain in north‐west China

Over the past 20 years, conservation tillage has been used on the loess plateau of north‐west China to improve the sustainability of local agriculture. There had been particular concern about loss of soil organic matter associated with traditional tillage. We examined the influence of four tillage treatments: conventional tillage (CT), subsoiling tillage (SST), rotary tillage (RT) and no‐tillage (NT), with two straw residue management treatments (return and removal) on the distribution with soil depth (0–20 cm, 20–40 cm) of total organic carbon, labile organic carbon (KMnO₄‐C) and bound organic carbon. The study was carried out on a Loutu soil (Earth‐cumuli‐Orthic Anthrosol) over seven consecutive years of a winter wheat (Triticum aestivum L.)–summer maize (Zea mays L.) crop rotation. By the end of this period, conservation tillage (SST, RT and NT) led to greater storage of soil organic carbon (SOC) (22.7, 14.9 and 16.3% with straw return in contrast to 21.4, 15.8 and 12.3% with no straw return, respectively) compared with CT in the surface soil (0–20 cm). The reduced tillage treatments (SST and RT) both increased significantly the highly labile organic carbon (HLOC) content of the surface soil (50% in both SST and RT) and mildly labile organic matter (MLOC) (49.4 in SST and 53.5% in RT) when straw was removed. The largest pool of bound carbon was observed in the Humin‐C pool, and the smallest in the free humic acids C (FHA‐C) in each tillage treatment. Conservation tillage led to an increased content of FHA‐C and CHA‐C. Results from correlation analyses indicate that SOC enrichment might have resulted from the increase in HLOC, MLOC, FHA‐C and CHA‐C over a short period. Labile organic carbon was associated with the organic carbon that was more loosely combined with clay (FHA‐C and CHA‐C). We conclude that both SST and RT are effective in maintaining or restoring organic matter in Loutu soils in this region, and the effect is greater when they are used in combination with straw return.     

Soil carbon improvement under long‐term (36 years) no‐till sorghum production in a sub‐tropical environment

Soil organic matter (SOM) is considered an important indicator of soil quality, which can be impacted by crop production practices such as tillage. In this study, two long‐term tillage regimes (conventional tillage [CT] and no tillage [NT], conducted for 36 years) were compared in continuous sorghum production in a sub‐tropical environment in southeast Texas. The positive effects of long‐term NT practice were more conspicuous at the soil surface compared with the deeper soil profiles. The SOC was greater (1.5 t C ha^?1 greater) in the NT system compared with the CT system. Results from an incubation study indicate that the rate of C‐min at 0–5 cm soil depth was significantly greater (164 μg of CO₂–C g^?1 of soil greater) in NT than that of CT, but this trend was reversed at 10–20 cm depth wherein the C‐min rates were 106 μg of CO₂–C g^?1 of soil greater in CT compared with NT, which is likely because of soil disturbance during the study. Soil cumulative CO₂‐C emissions were greater in the CT system (7.28 g m^?2) than in the NT system (5.19 g m^?2), which is primarily attributed to high soil temperature conditions in the CT system. Sorghum grain yield however was not influenced by the differences in SOC content in this long‐term experiment. Overall, the present study found that long‐term conservation tillage improved SOC stock and reduced carbon loss, thus had a positive impact on soil health and sustainability.     

In situ and potential CO2 evolution from a Fluventic Ustochrept in southcentral Texas as affected by tillage and cropping intensity

Quality of agricultural soils is largely a function of soil organic matter. Tillage and crop management impact soil organic matter dynamics by modification of the soil environment and quantity and quality of C input. We investigated changes in pools and fluxes of soil organic C (SOC) during the ninth and tenth year of cropping with various intensities under conventional disk-and-bed tillage (CT) and no tillage (NT). Soil organic C to a depth of 0.2 m increased with cropping intensity as a result of greater C input and was 10% to 30% greater under NT than under CT. Sequestration of crop-derived C input into SOC was 22±2% under NT and 9±4% under CT (mean of cropping intensities ± standard deviation of cropping systems). Greater sequestration of SOC under NT was due to a lower rate of in situ soil CO₂ evolution than under CT (0.22±0.03 vs. 0.27±0.06 g CO₂–C g⁻¹ SOC yr⁻¹). Despite a similar labile pool of SOC under NT than under CT (1.1±0.1 vs. 1.0±0.1 g mineralizable C kg⁻¹ SOC d⁻¹), the ratio of in situ to potential CO₂ evolution was less under NT (0.56±0.03) than under CT (0.73±0.08), suggesting strong environmental controls on SOC turnover, such as temperature, moisture, and residue placement. Both increased C sequestration and a greater labile SOC pool were achieved in this low-SOC soil using NT and high-intensity cropping.     

Carbon and nitrogen stocks in relation to organic matter fractions, aggregation and pore size distribution in no-tillage and conventional tillage in northern France

The magnitude of and mechanisms for long‐term differences in soil organic matter stocks under no‐tillage and conventional tillage are still relatively poorly known. We quantified differences in total C and N stocks after 32 years of no‐tillage (NT) and conventional tillage (CT) in plots with a long‐term cultivation history before differentiation and the same annual C and N returns to the soil. The role of physical protection of organic matter (OM) in these stock differences was further investigated by examining the changes at different levels of structural complexity, i.e. organic matter fractions, aggregation and pore‐size distribution. Four structural zones were sampled: loose and dense soil zones under CT and the 0–5 cm (rich in OM) and 5–20 cm (massive structure) soil layers under NT. The C and N stocks, calculated for an equivalent mass of dry soil, were only 10–15% larger under NT than under CT. Mineral‐associated N and particulate organic matter accounted for about 50% of the difference in N stocks. However, 66% of the total difference in C stocks was due to differences in the particulate organic matter (58%) and free residues (8%) fractions. The additional C and N under NT were almost exclusively situated in aggregates larger than 250 μm in diameter. Our results suggest that physical protection of OM under NT contributes significantly to the differences in C and N stocks between NT and CT by (i) enhanced macroaggregate formation in the 0–5 cm layer due to greater microbial activity and OM content and (ii) a better protection of soil organic matter in the 5–20 cm layer due to the presence of small pores and lack of soil disruption by tillage or climate.     

Tillage Effects on Particulate and Mineral‐Associated Organic Matter in Two Tropical Brazilian Soils

Abstract

Two Ferralsols (350 and 600 g kg^?1 clay) from the Brazilian Cerrado Region were evaluated for long‐term effects (5 and 8 years) of no tillage on carbon (C) stocks in particulate (>53 µm) and mineral‐associated (<53 µm) soil organic matter (SOM) fractions. Carbon stocks in particulate SOM increased under no tillage compared with conventional tillage, and the rate was higher in the clayey soil (0.62 Mg C ha^?1 yr^?1) than in the sandy clay loam soil (0.31 Mg C ha^?1 yr^?1). In contrast, the mineral‐associated SOM in the top soil layer (0–20 cm) was not affected by tillage system. Sequestration of atmospheric C in tropical no‐tillage soils seems to be due to accumulation of C in labile SOM fractions, with highest rates in clayey soils probably due to physical protection.     

Organic Farming Affects C and N in Soils Under Olive Groves in Mediterranean Areas

Under semiarid climatic conditions, intensive tillage increases soil organic matter losses, reduces soil quality, and contributes to climate change due to increased CO₂ emissions. There is a need for an agricultural management increasing soil organic matter. This paper presents the organic carbon (OC) and nitrogen (N) stocks, C:N ratio and stratification ratios (SRs) of these properties for olive groves soils under long‐term organic farming (OF), and conventional tillage (CT) in Los Pedroches valley, southern Spain. The results show that OF increased C and N stocks. The soil organic carbon (SOC) stock was 73·6 Mg ha⁻¹ in OF and 54·4 Mg ha⁻¹ in CT; and the total nitrogen (TN) stock was 7·1 Mg ha⁻¹ and 5·8 Mg ha⁻¹ for OF and CT, respectively. In the surface horizon (A: 0–16·9 cm in OF and Ap: 0–21·8 cm in CT) and Bw horizon (16·9–49·6 cm in OF and 21·8–56 cm in CT), SOC and TN concentrations and C:N ratios were higher in OF than in CT. Soil properties stratification in depth, expressed as a ratio, indicates the soil quality under different soil management systems. The SR of SOC ranged from 2·2 to 3·1 in OF and from 2·1 to 2·2 in CT. However, only SR2 (defined by Ap‐A/C) showed significant differences between CT and OF. The SR of TN showed similar trends to that of the SR of SOC. Organic farming contributes to a better soil quality and to increased carbon sequestration. Copyright © 2013 John Wiley & Sons, Ltd.     

Soil Nutrients Status after Fifty Years of Tillage and Nitrogen Fertilization

Long-term studies are valuable in assessing the impact of crop management practices on soil sustainability and function. This study used two calculation scenarios, fixed depth and Equivalent Soil Mass (ESM) to assess (i) soil nutrient status and (ii) soil organic carbon (SOC) after 50 years of nitrogen (N) fertilizer application rates (0, 22, 45, and 67 kg N ha^?1) and tillage [clean tillage (CT), reduced tillage (RT), and no-tillage (NT)] in a dryland winter wheat-sorghum-fallow cropping system. The soil organic matter (SOM) content increased by 33% with NT and RT compared with CT. The SOC at 0–30 cm was 39% greater than 30–60 cm depth with both fixed depth and ESM calculations. Soil nutrient specifically soil calcium (Ca), magnesium (Mg), and phosphorus (P) associated with N rates were no different than the control. Crop nutrient removal may eventually reduce soil nutrient contents with only N application. Nutrient addition specifically P should be considered in the future.     

Soil organic matter stratification ratio as an indicator of soil quality

Soil quality is a concept based on the premise that management can deteriorate, stabilize, or improve soil ecosystem functions. It is hypothesized that the degree of stratification of soil organic C and N pools with soil depth, expressed as a ratio, could indicate soil quality or soil ecosystem functioning, because surface organic matter is essential to erosion control, water infiltration, and conservation of nutrients. Stratification ratios allow a wide diversity of soils to be compared on the same assessment scale because of an internal normalization procedure that accounts for inherent soil differences. Stratification ratios of soil organic C were 1.1, 1.2 and 1.9 under conventional tillage (CT) and 3.4, 2.0 and 2.1 under no tillage (NT) in Georgia, Texas, and Alberta/British Columbia, respectively. The difference in stratification ratio between conventional and NT within an environment was inversely proportional to the standing stock of soil organic C to a depth of 15–20 cm across environments. Greater stratification of soil C and N pools with the adoption of conservation tillage under inherently low soil organic matter conditions (i.e., warmer climatic regime or coarse-textured soil) suggests that standing stock of soil organic matter alone is a poor indication of soil quality. Stratification of biologically active soil C and N pools (i.e., soil microbial biomass and potential activity) were equally or more sensitive to tillage, cropping intensity, and soil textural variables than stratification of total C and N. High stratification ratios of soil C and N pools could be good indicators of dynamic soil quality, independent of soil type and climatic regime, because ratios >2 would be uncommon under degraded conditions.     

Modeling organic carbon dynamics under no-tillage and plowed systems in tropical soils of Brazil using CQESTR

CQESTR simulates the effect of management practices on soil organic carbon (SOC) stocks. The beta version of the model had been calibrated and validated for temperate regions. Our objective was to evaluate the CQESTR model performance for simulating carbon dynamics as affected by tillage practices in two tropical soils (Ultisol and Oxisol) in southeastern and northeastern Brazil. In the southeast (20.75 S 42.81 W), tillage systems consisted of no tillage (NT); reduced tillage (RT) (one disc plow and one harrow leveling [RT1] or one heavy disc harrow and one harrow leveling [RT2]); and conventional tillage (CT) (two heavy disc harrows followed by one disc plow and two harrow levelings). In the northeast (7.55 S 45.23 W), tillage systems consisted of NT, RT (one chisel plow and one harrow leveling), and CT (one disk plow, two heavy disk harrowings, and two harrow levelings). CQESTR underestimated SOC at both sites, especially under NT systems, indicating that adjustments (e.g., the inclusion of clay mineralogy factor) are necessary for more accurate simulation of SOC in the tropics. In spite of this, measured and simulated values of SOC in the 0–20 cm depth were well correlated (southeast, R² = 0.94, p < 0.01; northeast, R² = 0.88, p < 0.05). With respect to initial conditions (native forest), CQESTR estimated a decrease in SOC stocks in plowed and no-tillage systems. In 2006, in the southeast, SOC stocks were 28.8, 23.7, 23.2, and 22.0 Mg ha^?1 under NT, RT2, RT1, and CT, respectively; in the northeast, stocks were 36.0, 33.8, and 32.5 Mg ha^?1 under NT, RT, and CT, respectively. The model estimated carbon emissions varying from 0.36 (NT) to 1.05 Mg ha^?1 year^?1 (CT) in the southeast and from 0.30 (NT) to 0.82 (CT) Mg ha^?1 year^?1 in the northeast. CQESTR prediction of SOC dynamics illustrates acceptable performance for the two tropical soils of Brazil.     

Soil organic carbon and nitrogen fractions and water‐stable aggregation as affected by cropping and grassland reclamation in an arid sub‐alpine soil

This paper investigates effects of cropping abandonment and perennial grass growing on soil organic C and N pools and aggregate stability, by comparing soils under native grassland, crop cultivation, perennial grass growing and cropping abandonment, in degraded cropland at a sub‐alpine site in north‐western China. The pools of total and particulate organic C (115 and 37 Mg ha⁻¹) in the 0–30 cm soil layer of native grassland were reduced by 31 and 54% after 30 years of crop cultivation. After 4 years of conversion from cropland to perennial grass growing total and particulate organic C pools were increased by 29 and 56%, whereas 4 year cropping abandonment increased particulate organic C by 36%. Rapid increases in total and particulate N were also found in perennial grass growing and cropping abandonment soils. The native grassland soil and soils of cropping abandonment and perennial grass growing had higher carbohydrate C concentrations in the 0–10 cm layer than the cropped soil. The rapid recovery of particulate organic fraction and carbohydrates in the re‐vegetated soils were probably due to higher plant biomass inputs and lower organic matter decomposition compared with those in the cropped soil. Aggregate stability of the 0–30 cm soil layer was significantly decreased by crop cultivation but showed a good recovery after 4 year re‐vegetations. This study suggests that reduction of soil organic matter and aggregate stability under crop cultivation may be remedied by cropping abandonment or perennial grass growing. Copyright © 2008 John Wiley & Sons, Ltd.     

Tillage effects on soil aggregation,organic carbon fractions and grain yield in Eum‐Orthic Anthrosol of a winter wheat–maize double‐cropping system,Northwest China

Conservation tillage has been applied in vast semi‐arid regions of the Guanzhong Plain, Northwest China. The tillage effects on soil aggregation, organic carbon (OC) stabilization and grain yield on this plain have not been fully elucidated. A 9‐year field experiment was established from 2002 on a silty clay loam soil (Eum‐Orthic Anthrosol) growing winter wheat–maize in a double‐cropping system. Six conservation tillage treatments were applied by different combinations of rotary tillage (RT), subsoiling (SS) and no‐till (NT), with or without finely chopped straw retention. Conventional tillage (CT) acted as the control. Results showed that in the surface (0–10 cm) soil, the proportion of water‐stable aggregates (WSA) <0.05 mm was 18% less while that for WSA >2 mm was 98% more under NT treatments compared with CT. Additionally, the oxidizable OC content in WSA 0.25–2 mm was 27% greater under NT treatments compared with CT. The OC stocks increased under SS by 17%, RT by 16% and NT by 15% relative to CT. Grain yield (wheat + maize) showed similar increasing trends in all the tillage treatments compared with CT. Both OC stocks and grain yield were larger in treatments with than without straw retentions. These results indicate that NT is beneficial for OC accumulation in WSA but is limited in its ability to improve soil structure in this region. SS plus straw retention (fine‐chopped or as a mulch) is an effective practice to improve soil structural stability, OC accumulation and soil productivity of Eum‐Orthic Anthrosols in Northwest China.     

Soil Quality Indicators as Affected by Shallow Tillage in a Vineyard Grown in a Semiarid Mediterranean Environment

Within the Mediterranean basin, soil tillage enhances the mineralisation of soil organic matter. We assessed the short‐term impact of shallow tillage [field cultivator (FC), rotary tiller (RT) and spading machine (SM)] on some soil quality indicators [bulk density, water‐stable aggregates, total and labile organic C pools (microbial biomass and extractable organic C), soil respiration and related eco‐physiological indexes] in a Sicilian vineyard. Also no tillage was included. We hypothesized that (i) RT and FC worsened soil quality indicators more than SM, and (ii) within the same tillage system, labile C pools, soil respiration and eco‐physiological indexes will respond more efficiently than chemical and physical soil properties since the tillage starts. The experiment started at March 2009, and each tillage type was applied three times per year (March or April, May and June), with soil tilled up to 15‐cm depth. Soil was sampled (0–15 and 15–30‐cm depth) in March 2009, April 2010, May 2012 and June 2014. SM was very effective in preserving soil organic matter pool and in improving any monitored soil quality indicator, similarly to no tillage. By contrast, RT was the most deleterious machine as it worsened most investigated indicators. Such deleterious effects were due to drastic disruption of soil aggregates and consequent exposition of protected soil organic matter to further microbial mineralization. Labile organic C pools and microbial quotients were the most responsive soil parameters for assessing the impact of shallow tillage on soil quality, even in the short term (<5 years). Copyright © 2016 John Wiley & Sons, Ltd.     

Soil organic carbon and fractions of a Rhodic Ferralsol under the influence of tillage and crop rotation systems in southern Brazil

Soil organic matter (SOM) and its different pools have key importance in optimizing crop production, minimizing negative environmental impacts, and thus improving soil quality. The objective of this study was to evaluate the soil C and N contents in bulk soil and in different SOM pools (light and heavy fractions) of a clayey Rhodic Ferralsol after 13 years of different tillage and crop rotations in Passo Fundo, State of Rio Grande do Sul, Brazil. Soil samples were collected from no-tillage (no soil disturbance except for sowing; NT) and conventional tillage (disc plough followed by light disc harrowings; CT) applied to wheat/soybean (W/S) and wheat/soybean–vetch/maize (W/S–V/M) rotations. As reference, soil was sampled from a non-cultivated area adjacent to the field experiment. The greatest soil C and N contents were found in non-cultivated soils in the 0–5 cm depth (45 g C kg⁻¹ soil and 3.6 g N kg⁻¹ soil). Crop cultivation led to a decrease in SOM content which was higher for CT soils (approx. 60% decrease in C and N contents) than NT soils (approx. 43% decrease in C and N contents) at 0–5 cm. Tillage had the greatest impact on soil C and N storage. Soils under NT did not contain higher C and N storage than CT soils below 5 cm depth. Significantly, higher amounts of organic carbon of FLF in CT (0.5–0.7 g C kg⁻¹ soil) than in NT soils (0.2 g C kg⁻¹ soil) at 10–20 cm depth were also observed and the differences in C and N storage between CT and NT soils in the 0–30 cm layer were not significant. Silt and clay fractions contained the largest amount of organic carbon (60–95% of total organic carbon), and free light fraction was the most sensitive pool of organic carbon to detect changes in SOM due to soil tillage and crop rotations.     

Soil carbon and nitrogen dynamics as affected by long-term tillage and nitrogen fertilization

Quantifying seasonal dynamics of active soil C and N pools is important for understanding how production systems can be better managed to sustain long-term soil productivity especially in warm subhumid climates. Our objectives were to determine seasonal dynamics of inorganic soil N, potential C and N mineralization, soil microbial biomass C (SMBC), and the metabolic quotient of microbial biomass in continuous corn (Zea mays L.) under conventional (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no-tillage (NT) with low (45kgNha^–1) and high (90kgNha^–1) N fertilization. An Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas, United States, was sampled before corn planting in February, during pollination in May, and following harvest in July. Soil inorganic N, SMBC, and potential C and N mineralization were usually highest in soils under NT, whereas these characteristics were consistently lower throughout the growing season in soils receiving MB tillage. Nitrogen fertilization had little effect on soil inorganic N, SMBC, and potential C and N mineralization. The metabolic quotient of microbial biomass exhibited seasonal patterns inverse to that of SMBC. Seasonal changes in SMBC, inorganic N, and mineralizable C and N indicated the dependence of seasonal C and N dynamics on long-term substrate availability from crop residues. Long-term reduced tillage increased soil organic matter (SOM), SMBC, inorganic N, and labile C and N pools as compared with plowed systems and may be more sustainable over the long term. Seasonal changes in active soil C and N pools were affected more by tillage than by N fertilization in this subhumid climate. Received: 20 September 1996     

Is it necessary to adjust nitrogen recommendations for tillage and wheat residue management in irrigated sweet corn?

ABSTRACT

Conservation tillage practices have gained interests. A 2-year field study (2014–2015) was conducted to evaluate four N rates (0, 69, 138, and 207 kg N ha^?1) effects on irrigated sweet corn (Zea mays L.) grown with or without wheat (Triticum aestivum L.) residue removal and conventional (CT), reduced (RT), or no-tillage (NT) practices near Shiraz, Iran. After 2 years, maximum marketable yield occurred at 156 and 159 kg N ha^?1 under CT and NT, respectively, while yield was tended to be increased with increasing N rates under RT. Increasing N rate increased total plant N uptake, shoot, and grain N accumulation. The lowest nitrogen use efficiency (NUE) was obtained under NT, while RT and CT either showed similar effects or RT was superior over CT. Soil total N was greater under CT and residue retention showed 18% and 14% higher N concentration than residue removal in 2014 and 2015, respectively. Soil organic matter was the highest (2.59%) under RT with residue retention and 138 kg N ha^?1. Conservation tillage needs more N rather than CT during transition from conventional to conservation agriculture practices, but it is based on the short-term results and evaluation of long-term experiment is highly recommended.     

Nitrogen cycling in organic farming systems with rotational grass–clover and arable crops

Organic farming is considered an effective means of reducing nitrogen losses compared with more intensive conventional farming systems. However, under certain conditions, organic farming may also be susceptible to large nitrogen (N) losses. This is especially the case for organic dairy farms on sandy soils that use grazed grass–clover in rotation with cereals. A study was conducted on two commercial organic farms on sand and loamy sand soils in Denmark. On each farm, a 3‐year‐old grass–clover field was selected. Half of the field was ploughed the first year and the other half was ploughed the following year. Spring barley (Hordeum vulgare L.) was sown after ploughing in spring. Measurements showed moderate N leaching during the pasture period (9–64 kg N ha^?1 year^?1) but large amounts of leaching in the first (63–216 kg N ha^?1) and second (61–235 kg N ha^?1) year after ploughing. There was a small yield response to manure application on the sandy soil in both the first and second year after ploughing. To investigate the underlying processes affecting the residual effects of pasture and N leaching, the dynamic whole farm model farm assessment tool (FASSET) was used to simulate the treatments on both farms. The simulations agreed with the observed barley N‐uptake. However, for the sandy soil, the simulation of nitrate leaching and mineral nitrogen in the soil deviated considerably from the measurements. Three scenarios with changes in model parameters were constructed to investigate this discrepancy. These scenarios suggested that the organic matter turnover model should include an intermediate pool with a half‐life of about 2–3 years. There might also be a need to include effects of soil disturbance (tillage) on the soil organic matter turnover.     

Budgets for root-derived C and litter-derived C: comparison between conventional tillage and no tillage soils

Placement of plant residues in conventional tillage (CT) and no-tillage (NT) soils affects organic matter accumulation and the organization of the associated soil food webs. Root-derived C inputs can be considerable and may also influence soil organic matter dynamics and soil food web organization. In order to differentiate and quantify C contributions from either roots or litter in CT and NT soils, a ¹⁴C tracer method was used.To follow root-derived C, maize plants growing in the field were ¹⁴C pulse-labeled, while the plant litter in those plots remained unlabeled. The ¹⁴C was measured in NT and CT soils for the different C pools (shoots, roots, soil, soil respiration, microbial biomass). Litter-derived C was followed by applying ¹⁴C labeled maize litter to plots which had previously grown unlabeled maize plants. The ¹⁴C pools measured for the litter-derived CT and NT plots included organic matter, microbial biomass, soil respiration, and soil organic C.Of the applied label in the root-derived C plots, 35–55, 6–8, 3, 1.6, and 0.4–2.4% was recovered in the shoots, roots, soil, cumulative soil respiration, and microbial biomass, respectively. The ¹⁴C recovered in these pools did not differ between CT and NT treatments, supporting the hypothesis that the rhizosphere microbial biomass in NT and CT may be similar in utilization of root-derived C. Root exudates were estimated to be 8–13% of the applied label. In litter-derived C plots, the percentage of applied label recovered in the particulate organic matter (3.2–82%), microbial biomass (4–6%), or cumulative soil respiration (12.5–14.7%) was the same for CT and NT soils. But the percentage of ¹⁴C recovered in CT soil organic C (18–69%) was higher than that in NT (12–43%), suggesting that particulate organic matter (POM) leaching and decomposition occurred at a higher rate in CT than in NT. Results indicate faster turnover of litter-derived C in the CT plots.     

Effects of 30 years of cropping and tillage systems on surface soil test changes

Abstract

A common belief is that no‐till systems with adequate fertility will improve soil quality over other tillage systems. The objectives of this study were to determine if crop phase, tillage systems, and n rate in a long‐term rotation affected soil chemical analyses in the surface 15 cm of soil and to evaluate the trend in chemical analyses. To test this hypothesis, surface soil samples were taken from a long‐term (30‐year) cropping and tillage study. This study was initiated in 1965 on a Harney silt loam soil in Central Kansas with every phase of the wheat‐sorghum‐fallow (WDF) rotation included each year. Tillage systems included clear‐till (CT), reduced‐till (RT), and no‐till (NT). In 1975, four nitrogen (N) rates (0, 22, 45, 67 kg N ha¹) were incorporated by subdividing the tillage plots. Topdressed N, as ammonium nitrate, was the only fertilizer added throughout the duration of the study. Soil samples were taken at depths of 0 to 7.5 and 7.5 to 15 cm in all plots in 1965 and in 1995. In 1998, soils on 1997 sorghum plots were samples in 2.5‐cm increments to 15 cm. Samples from all dates were analyzed for pH, available phosphorus (AP), and organic matter (OM), and deviations from the controls from 1965 to 1995 were assessed by subtracting 1995 results from 1965 results. The change in soil pH showed a crop phase by sample depth interaction. In the wheat phase, pH in the top 7.5 cm increased by 0.19 and increased by 0.28 in the 7.5–15 cm layer. In the fallow phase, pH increased by 0.04 and 0.35 in the top 7.5 cm and 7.5–15 cm layers, respectively. The pH change for sorghum was intermediate for both depths. The increase in overall pH from 1965 to 1995 was unexpected and contrary to normal expectations of a decrease over time. Soil OM was not changed significantly over the 30 years of the study, suggesting that OM buildup or depletion is very slow under this cropping system on a nearly level soil with minimal soil erosion. Increasing the rate of N application significantly reduced pH in the upper increment samples, but had little effect on pH below 10 cm. The NT system had the lowest surface increment in pH, but differences among tillage systems were minimal below 7.6 cm. The AP was highest for NT in the surface increment, but for CT at deeper depths. Likewise, OM was highest for NT in the 2.5 cm increment and the CT at deeper increments. Under the present N management, pH may reach levels where herbicide effectiveness and phosphorus availability could be affected adversely. Deep tillage by one‐way or mold‐board plowing might be an interim solution to raise the pH before liming is implemented or P fertilizer is added to maintain adequate AP throughout the top 15 cm. Nitrogen management may need to be changed to some form of band‐type placement to reduce the total N applied. Under the conditions of this study (WSF, reduced tillage, and 57 cm annual precipitation), soil OM increased very slowly.     

Simulating trends in soil organic carbon of an Acrisol under no-tillage and disc-plow systems using the Century model

Soil organic matter (SOM) and its different pools have key importance in nutrient availability, soil structure, in the flux of trace gases between land surface and the atmosphere, and thus improving soil health. This is particularly critical for tropical soils. The rates of accumulation and decomposition of carbon in SOM are influenced by several factors that are best embodied by simulation models. However, little is known about the performance of SOM simulation model in an acid tropical soil under different tillage systems including no-tillage (NT). Our objective was to simulate soil organic matter dynamics on an Acrisol under no-tillage and different plowed systems using Century model. Tillage systems consisted of no-tillage, disc plow, heavy disc harrow followed by disc plow, and heavy disc harrow. Soil C stocks simulated by Century model showed tendency to recovery only under no-tillage. Also, simulated amounts of C stocks of slow and active pools were more sensitive to management impacts than total organic C. The values estimated by Century of soil C stocks and organic carbon in the slow and passive pools fitted satisfactorily with the measured data. Thus fitted, except for the active pool, Century showed acceptable performance in the prediction of SOM dynamics in an acid tropical soil.