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.     

Cultivation effects on temporal changes of organic carbon and aggregate stability in desert soils of Hexi Corridor region in China

Sustainable agricultural use of cultivated desert soils has become a concern in Hexi Corridor in Gansu Province of China, because loss of topsoil in dust storms has been recently intensified. We chose four desert sites to investigate the effects of cultivation (cropping) on (i) soil organic C and its size fractions and (ii) soil aggregate stability (as a measure of soil erodibility). These parameters are of vital importance for evaluating the sustainability of agricultural practices.

Total organic C as well as organic C fractions in soil (coarse organic C, 0.1–2 mm; young organic C, 0.05–0.1 mm; stable organic C, <0.05 mm) generally increased with the duration of the cultivation period from 0 (virgin soil, non-cultivated) to more than 30 years (p < 0.05). Compared to total organic C in virgin soils (2.3–3.5 g kg⁻¹ soil), significantly greater values were found after 10 to >20 years of cultivation (6.2–7.1 g kg⁻¹ soil). The increase in organic C in desert soils following prolonged cultivation was mainly the consequence of an increase in the coarse organic C. The increase in total organic C in soil was also dependent on clay content [total organic C = 0.96 + 0.249 clay content (%) + 0.05 cultivation year, R² = 0.48, n = 27, p < 0.001]. This indicates that clay protected soil organic C from mineralization, and also contributed to the increase in soil organic C as time of cultivation increased.

There was a significant positive correlation between aggregate stability and total organic C across all field sites. The water stability of aggregates was low (with water-stable aggregate percentage 4% of dry-sieved aggregates of size 1–5 mm). There was no consistent pattern of increase in the soil aggregate stability with time of cultivation at different locations, suggesting that desert soils might remain prone to wind erosion even after 50 years of cultivation. Alternative management options, such as retaining harvested crop residues on soil surface and excluding or minimizing tillage, may permit sustainable agricultural use of desert soils.     

Influence of agricultural land management on organic matter content,microbial activity and aggregate stability in the profiles of two Oxisols

The effects of agricultural land use on organic matter content and related soil microbial and physical properties were compared with those under undisturbed native grassland in KwaZulu-Natal, South Africa. Two separate farms situated on Oxisols were used and both contained fields with continuous long-term (>20 y) cropping histories. At site 1, soil organic C content in the surface 30 cm followed the order permanent kikuyu pasture > annual ryegrass pasture > native grassland > sugarcane > maize under conventional tillage (CT). At site 2, organic C in the surface 30 cm decreased in the order kikuyu pasture > native grassland > annual ryegrass pasture > maize under zero tillage (ZT) > maize CT. Organic C, microbial biomass C, percentage organic C present as organic C, basal respiration and aggregate stability were substantially greater in the surface 5 cm under maize ZT than maize CT but this trend tended to be reversed in the 10- to 30-cm layer. In the undisturbed sites (e.g. native grassland and kikuyu pasture) the metabolic quotient increased with depth. By contrast, under maize CT and sugarcane there was no significant stratification of organic C, yet there was a sharp decrease in the metabolic quotient with depth. Aggregate stability was high under both native grassland and kikuyu pasture and it remained high to 40 cm depth under the deep-rooted kikuyu pasture. Although soil organic C content was similar under maize CT and sugarcane, values for microbial biomass C, percentage of organic present as microbial biomass, basal respiration and aggregate stability were lower, and those for metabolic quotient and bulk density were higher, under sugarcane. This was attributed to the fallow nature of the soil in the interrows of sugarcane fields. It was concluded that the loss of organic matter, microbial activity and aggregate stability is potentially problematic under maize CT, sugarcane and annual pasture and measures that improve organic matter status should be considered.     

Impact of cultivation and sugar-cane green trash management on carbon fractions and aggregate stability for a Chromic Luvisol in Queensland, Australia

Technological advances in sugar-cane harvesting and processing is bringing about rapid changes in production systems which could impact on soil physical conditions. An increasing incidence of soil structural decline and depletion of soil carbon levels has increased the risk of soil erosion and crop yield reductions. Soil carbon (C) and aggregate stability were studied on a sugar-cane (Saccharum officinarum L.) green trash blanket trial that had been established on a Chromic Luvisol soil at Mackay, Qld, Australia in 1992. The experiment consisted of blocks with two blocks being harvested early and the remaining two blocks harvested late in the crushing season. Within each block, treatment combinations of trash burnt or green trash blanket, which are either cultivated between rows or not cultivated after harvest, were included. Cropping and cultivation of the soil reduced the different C fractions in the surface 0–100 mm layer by 66–67% when compared to an adjacent uncropped reference soil. The labile C (C_L) concentration was 11% lower in the burnt treatment compared to the trash returned treatment but the opposite was found for total C (C_T). After four years, the no cultivation treatment had higher concentrations of all C fractions measured, compared to the cultivated treatment. When compared to the uncropped reference soil, cropping resulted in marked reductions in aggregate mean weight diameter (MWD) and aggregates >250 μm and an increase in aggregates <125 μm determined by both immersion and tension wetting. The return of the green trash resulted in a 30% greater MWD and a 28% increase in aggregates >250 μm and an 18% reduction in aggregates <125 μm compared to the burnt treatment when immersion wetting was used. Four years of cultivation reduced the MWD, as determined by immersion wetting, by 26% compared to the no cultivation treatment. No significant correlations were found between any measured C fraction and aggregate stability. This study indicates that sustainable sugar-cane cropping systems will likely be those where cultivation is kept to a minimum and trash is retained in the system.     

Depth distribution of soil organic C and N after long-term soybean cropping in Texas

Crop management practices have potential to enhance subsoil C and N sequestration in the southern U.S., but effects may vary with tillage regime and cropping sequence. The objective of this study was to determine the impacts of tillage and soybean cropping sequence on the depth distribution of soil organic C (SOC), dissolved organic C (DOC), and total N after 20 years of treatment imposition for a silty clay loam soil in central Texas. A continuous soybean monoculture, a wheat–soybean doublecrop, and a sorghum–wheat–soybean rotation were established under both conventional (CT) and no tillage (NT). Soil was sampled after soybean harvest and sectioned into 0–5, 5–15, 15–30, 30–55, 55–80, and 80–105 cm depth intervals. Both tillage and cropping intensity influenced C and N dynamics in surface and subsurface soils. No tillage increased SOC, DOC, and total N compared to CT to a 30 cm depth for continuous soybean, but to 55 cm depths for the more intensive sorghum–wheat–soybean rotation and wheat–soybean doublecrop. Averaged from 0 to 105 cm, NT increased SOC, DOC, and total N by 32, 22, and 34%, respectively, compared to CT. Intensive cropping increased SOC and total N at depths to 55 cm compared to continuous soybean, regardless of tillage regime. Continuous soybean had significantly lower SOC (5.3 g kg⁻¹) than sorghum–wheat–soybean (6.4 g kg⁻¹) and wheat–soybean (6.1 g kg⁻¹), and 19% lower total N than other cropping sequences. Dissolved organic C was also significantly higher for sorghum–wheat–soybean (139 mg C kg⁻¹) than wheat–soybean (92 mg C kg⁻¹) and continuous soybean (100 mg C kg⁻¹). The depth distribution of SOC, DOC, and total N indicated treatment effects below the maximum tillage depth (25 cm), suggesting that roots, or translocation of dissolved organic matter from surface soils, contributed to higher soil organic matter levels under NT than CT in subsurface soils. High-intensity cropping sequences, coupled with NT, resulted in the highest soil organic matter levels, demonstrating potential for C and N sequestration for subsurface soils in the southern U.S.     

严重退化红壤植被恢复后有机质富集和团聚体稳定性

Three types of soils： an eroded barren soil under continuous fallow, an eroded soil transplanted with Lespedeza shrubs （Lespedeza bieolor）, and an eroded soil transplanted with camphor tree （Cinnaraomum camphora） were investigated to quantify organic matter pools and aggregates in reforested soils using physical fractionation techniques and to determine aggregate stability in relation to the enrichment of soil organic carbon （SOC）. Soil organic matter （SOM） was physically fractionalized into free particulate organic matter （fPOM）, occluded particulate organic matter （oPOM）, and mineralassociated organic matter （mOM）. The SOM was concentrated on the surface soil （0 5 cm）, with an average C sequestration rate of 20-25 g C m^-2 year^-1 over 14 years. As compared to the eroded barren land, organic C content of fPOM, oPOM, and mOM fractions of the soil under Lespedeza and under camphor tree increased 12-15, 45-54, and 3.1-3.5 times, respectively. A linear relationship was found between aggregate stability and organic C （r^2 = 0.45, P 〈 0.01）, oPOM （r^2 = 0.34, P 〈 0.05）, and roOM （r^2 = 0.46, P 〈 0.01） of aggregates. The enrichment of organic C improved aggregate stability of the soil under Lespedeza but not that under camphor tree. However, further research is needed on the physical and biological processes involved in the interaction of soil aggregation and SOC sequestration in ecosystem.     

Soil C and N changes on conservation reserve program lands in the Central Great Plains

The Conservation Reserve Program (CRP) was initiated to reduce water and wind erosion on marginal, highly erodible croplands by removing them from production and planting permanent, soil-conserving vegetation such as grass. We conducted a field study at two sites in Wyoming, USA, in order to quantify changes in soil C and N of marginal croplands seeded to grass, and of native rangeland plowed and cropped to wheat–fallow. Field plots were established on a sandy loam site and a clay loam site on wheat–fallow cropland that had been in production for 60+ years and on adjacent native rangeland. In 1993, 6 years after the study was initiated, the surface soil was sampled in 2.5 cm depth increments, while the subsurface soil was composited as one depth increment. All soil samples were analyzed for total organic C and N, and potential net mineralized C and N. After 60+ years of cultivation, surface soils at both study sites were 18–26% lower (by mass) in total organic C and N than in the A horizons of adjacent native range. Six years after plowing and converting native rangeland to cropland (three wheat–fallow cycles), both total and potential net mineralized C and N in the surface soil had decreased and NO₃–N at all depths had increased to levels found after 60+ years of cultivation. We estimate that mixing of the surface and subsurface soil with tillage accounted for 40–60% of the decrease in surface soil C and N in long-term cultivated fields; in the short-term cultivated fields, mixing with tillage may have accounted for 60–75% of the decrease in C, and 30–60% of the decrease in N. These results emphasize the need to evaluate C and N in the entire soil solum, rather than in just the surface soil, if actual losses of C and N due to cultivation are to be distinguished from vertical redistribution. Five years after reestablishing grass on the sandy loam soil, both total and potential net mineralized C and N in the surface soil had increased to levels equal to or greater than those observed in the A horizon of the native range. On the clay loam soil, however, significant increases in total organic C were observed only in the surface 2.5 cm of N-fertilized grass plots, while total organic N had not significantly increased from levels observed in the long-term cultivated fields.     

砂壤土上甘蔗管理的土壤质量指示评估

An important factor for the sustainability of soils highly susceptible to degradation is the use of monitoring tools that promptly and realistically reflect changes imposed on soil by different cropping systems. To select soil quality indicator variables in sugarcane (Saccharum officinarum L.) production areas that fulfill the criteria of sensitivity to management practices and between-season consistency in the management discrimination, ten composite soil samples (0-10 cm) were collected in July 2005 (rainy season) and again in March 2006 (dry season) from areas under cultivation of organic sugarcane (OS), green sugarcane (GS), burned sugarcane (BS) and from an adjacent native forest (NF) area at Usina Triunfo, Boca da Mata, Alagoas, Brazil. Microbial biomass-C (MBC), total organic C (TOC), soil enzyme activity expressed as the rate of fluorescein diacetate (FDA) hydrolysis, mean weight diameter of water-stable soil aggregates (MWD), and percentage of water-stable macroaggregates (PWSA) were analyzed. Although MBC and TOC were higher in NF than in the cultivated areas, no differences were observed in these C pools between the three sugarcane systems. The response of FDA to the site management was dependent on the sampling time. In the rainy period, the activity followed the order: NF > OS > GS > BS, whereas in the dry season, only NF differed from the other treatments. Irrespective of the sampling time, MWD and PWSA decreased in the order NF > OS = GS > BS. The variables MWD and PWSA are quite sensitive for discriminating between site management histories regardless the sampling season.     

复种模式对豫西褐土团聚体稳定性及其碳、氮分布的影响

  【目的】  长期单一的玉米–小麦复种模式会引起土壤结构破坏、农田生产力下降。探究不同复种模式对农田土壤团聚体稳定性及其碳、氮分布的影响,为维持土壤结构稳定,实现农业可持续发展提供科学依据。  【方法】  定位试验在河南洛阳褐土上进行。设置冬小麦–夏玉米 (T1)、冬小麦–夏花生 (T2)、冬小麦–夏玉米||花生间作 (2行玉米间作4行花生,T3) 3个复种模式处理。试验始于2014年6月,2019年10月夏季作物收获后 (共11茬作物),采集0—20和20—40 cm土层土壤样品,利用湿筛法和干筛法分析土壤团聚体组成、团聚体稳定性、有机碳和全氮在不同粒级团聚体中的含量及分配比例。  【结果】  与T1相比,在0—20 cm土层中,T2和T3处理土壤中 > 0.25 mm粒级的机械性团聚体 (DR_0.25) 占比分别增加了5.9%和9.9%,> 0.25 mm粒级的水稳性团聚体 (WR_0.25) 占比分别增加了50.3%和57.9%,不稳定性团粒指数 (E_LT) 分别较T1减少了33.2%和50.6%,土壤团聚体破坏率 (PAD) 分别较T1减少49.3%和51.4%,土壤团聚体平均质量直径 (MWD) 分别较T1增加36.4%和47.0%,几何平均直径 (GMD) 分别较T1增加100.0%和120.0%。在20—40 cm土层中,T2和T3处理土壤中不稳定性团粒指数 (E_LT) 分别较T1减少了13.2%和18.0%,土壤团聚体破坏率 (PAD) 分别较T1减少21.4%和28.8%,土壤团聚体平均质量直径 (MWD) 分别较T1增加4.8%和6.0%,几何平均直径 (GMD) 分别较T1增加11.5%和7.7%。各粒级的有机碳和全氮含量均以2～0.25 mm粒级最高,且0—20 cm土层的有机碳和全氮含量高于20—40 cm土层。与T1处理相比,T3处理显著提高了0—20 cm土层各粒级土壤全氮的贡献率和 > 0.25 mm粒级土壤有机碳的贡献率,降低了 < 0.25 mm粒级土壤有机碳的贡献率;T2处理显著提高了0—20 cm土层除 >2和<0.053 mm粒级外的土壤全氮的贡献率和 > 0.25 mm粒级土壤有机碳的贡献率,降低了0.25～0.053 mm粒级土壤有机碳的贡献率。与T1处理相比,T2和T3处理提升了0—20和20—40 cm土层土壤总有机碳、全氮、有效磷和速效钾的含量,3个处理的土壤容重和pH无显著差异。  【结论】  冬小麦–夏花生、冬小麦–夏玉米||花生复种模式较传统冬小麦–夏玉米复种模式明显增加了土壤大团聚体含量,增强了团聚体的机械稳定性和水稳定性,还可显著提高土壤团聚体 (特别是 > 0.25 mm粒级团聚体) 的碳、氮含量,提高土壤有效磷和速效钾含量,更有利于豫西褐土区农田土壤肥力保持。且冬小麦–夏玉米||花生复种的效果优于冬小麦–夏花生复种。     

Changes in the properties of a Mexican Fluvisol following 30 years of sugarcane cultivation

A study to evaluate the changes in the fertility of Fluvisols under continuous cultivation of sugarcane was conducted in the vicinity of the Santa Rosalía sugar refinery. About 4000 ha of Fluvisols have been under sugarcane cultivation for more than 30 years. Three representative plots under monoculture for 5, 10, 20 and 30 years, were selected under on-farm conditions. Several samples were obtained from crop row and inter-row zones, at 0–30 cm soil depth, and composited into four samples for each plot. Soil samples were analyzed for pH, organic matter, total organic C, total nitrogen, available phosphorus, exchangeable potassium, calcium and magnesium, cation exchange capacity, texture and bulk density. There were significant changes in the soil chemical properties due to 30 years of monoculture, except for OM, exchangeable K, Ca, Mg and CEC. Total soil organic C, total N and P declined by 17, 21 and 37%, respectively, by 30 years of monoculture of sugarcane. These losses mainly occurred during the first 20 years of cultivation. However, the adverse effect of decline in soil fertility was not reflected in the sugarcane yields. The 67% increase in yields observed between 5 and 30 years of monoculture may be attributed to the adoption of recommended cultural practices and to farmer's experience. Soil physical properties were not affected by the intensive sugarcane cultivation. A complete adoption of recommended practices by SR among farmers and a revision of the N and P fertilization rate are necessary to maintain the fertility of sugarcane soils.     

Fire and Tillage as Degrading Factors of Soil Structure in Northern Zagros Oak Forest,West Iran

The effects of fire and the conversion to vineyard on soil organic carbon (SOC), and soil aggregate size distribution and stability were studied in a forest of Iran. For this purpose, topsoil was sampled in an unburned area, a portion of the forest burned three years earlier, and a vineyard, all three contiguous and showing similar topographic features. In the burned forest, soil was sampled in areas undergone high, moderate, or low severity. Air‐dried soil samples were sieved to obtain four aggregate size classes, which were subsequently wet sieved. Soil aggregate distribution index, mean weight diameter, geometric mean diameter, and aggregate stability index were determined on both dry and wet specimens. No significant differences in SOC between burned and unburned forest were found, most probably because of the supply of charred biomass to soil, while in the vineyard thirty years of cultivation had removed half of initial SOC. Both severe fire and cultivation had decreased the stability of aggregates and the relative amount of the biggest ones (8 to 2‐cm diameter). However, aggregate stability was significantly lower in the vineyard than in the burned forest, which points out to a stronger impact of prolonged cultivation than a single fire, although severe. Cultivation and severe fire had decreased the proportion of C in macroaggregates, to the advantage of meso (1 to 0.25 mm) and micro (<0.25 mm) aggregates. A hierarchical cluster analysis of all investigated properties and indices demonstrated that cultivation and highly severe fire both were causes of soil degradation. Copyright © 2016 John Wiley & Sons, Ltd.     

Contrasting response to cropping of populations of earthworms and predacious nematodes in four soils

The activities of many soil animals make a positive contribution to soil processes and they should be considered for inclusion in indices of ‘soil quality'. To assess the potential use of nematodes and earthworms as indicators, the relationships between populations of earthworms (Lumbricidae), total number of nematodes and predacious nematodes (Mononchoidea) and six soil physical factors, soil carbon and pH were investigated in four New Zealand soils. In each, soil treatments ranged from 5–90 year pastures to cropping with maize or barley for 11–29 years. With increasing cultivation, trends in bulk density, total porosity, aggregate stability and concentration of total carbon were similar in all four soils. In Manawatu (Dystric Fluventic Eutrochrept) and Kairanga (Typic Endoaquept) soils earthworm populations were negligible under continuous cropping while Mononchoidea were abundant (11 600 and 34 100 m⁻²). In contrast, in Moutoa (Fluvaquentic Endoaquoll) and Wakanui (Aquic Ustochrept) soils earthworms persisted under cultivation, while Mononchoidea were less abundant (300 and 2500 m⁻²). At these two latter sites, aggregate stability was higher (1.14 and 0.92 mm mean weight diameter (MWD)) than in Manawatu and Kairanga soils (0.38 and 0.35 mm MWD). These relationships between aggregate stability, earthworm abundance and predacious nematodes show not only that some potential indicators may have a local rather than national application, but also that there are important interactions between soil physical properties and soil fauna which require further investigation.     

Long-term fertilization, manure and liming effects on soil organic matter and crop yields

Yield decline or stagnation and its relationship with soil organic matter fractions in soybean (Glycine max L.)–wheat (Triticum aestivum L.) cropping system under long-term fertilizer use are not well understood. To understand this phenomenon, soil organic matter fractions and soil aggregate size distribution were studied in an Alfisol (Typic Haplustalf) at a long-term experiment at Birsa Agricultural University, Ranchi, India. For 30 years, the following fertilizer treatments were compared with undisturbed fallow plots (without crop and fertilizer management): unfertilized (control), 100% recommended rate of N, NP, NPK, NPK+ farmyard manure (FYM) and NPK + lime. Yield declined with time for soybean in control (30 kg ha⁻¹ yr⁻¹) and NP (21 kg ha⁻¹ yr⁻¹) treatments and for wheat in control (46 kg ha⁻¹ yr⁻¹) and N (25 kg ha⁻¹ yr⁻¹) treatments. However, yield increased with time for NPK + FYM and NPK + lime treatments in wheat. At a depth of 0–15 cm, small macroaggregates (0.25–2 mm) dominated soil (43–61%) followed by microaggregates (0.053–0.25 mm) with 13–28%. Soil microbial biomass carbon (SMBC), nitrogen (SMBN) and acid hydrolysable carbohydrates (HCH) were greater in NPK + FYM and NPK + lime as compared to other treatments. With three decades of cultivation, C and N mineralization were greater in microaggregates than in small macroaggregates and relatively resistant mineral associated organic matter (silt + clay fraction). Particulate organic carbon (POC) and nitrogen (PON) decreased significantly in control, N and NP application over fallow. Results suggest that continuous use of NPK + FYM or NPK + lime would sustain yield in a soybean–wheat system without deteriorating soil quality.     

Organic matter status and the size,activity and metabolic diversity of the soil microflora as indicators of the success of rehabilitation of mined sand dunes

The effectiveness of the rehabilitation of mined sand dunes on the northern coast of KwaZulu–Natal, South Africa, was assessed based on measurements of the total and labile organic matter content and the size, activity and metabolic diversity of the soil microflora. Soil was sampled (0–10 cm) after 0, 5, 10, 20 and 25 years of rehabilitation and compared with soil under undisturbed native forest and under long-term commercial pine forest. Following topsoil removal, stockpiling and respreading on reformed dunes, there was a massive loss of organic C such that, at time zero, organic C content was only 24% of that present under native forest. Soil organic C content increased progressively during rehabilitation until, after 25 years, it represented 93% of that present under native forest. The pattern of change in light-fraction C, KMnO₄-extractable C, water-soluble C, microbial biomass C, basal respiration and arginine ammonification rate was broadly similar to that for organic C, but the extent of the initial loss and the magnitude of the subsequent increase differed. Microbial biomass C, water-soluble C and KMnO₄-extractable C, expressed as a percentage of organic C, declined during rehabilitation as humic substances progressively accumulated. Principal component (PC) analysis of catabolic response profiles to 36 substrates revealed that the catabolic diversity of microbial communities differed greatly between native forest, commercial pine forest, 0 years and 10 years of rehabilitation. On the PC1 axis, values for soils under native forest and after 25 years rehabilitation were similar, but there was still separation on the PC2 axis. The main factor explaining variation in response profiles on the PC1 axis was organic C content; and the greatest catabolic diversity occurred in soils under native forest and after 25 years of rehabilitation.     

Loss of soil organic matter and related soil properties under long-term sugarcane production on two contrasting soils

The effects of increasing periods under sugarcane monoculture (managed with preharvest burning) on soil organic matter content and related soil properties were investigated in the 0- to 10-cm layer of a sandy coastal Ochric Cambisol (Glenrosa soil) and a red Rhodic Ferralsol (Hutton soil) from the sugar belt of the South African province of KwaZulu-Natal. The organic C content at both sites under undisturbed vegetation was between 40 and 50 g C kg^-1. This declined exponentially with increasing years under sugarcane. For the Glenrosa site it reached a new equilibrium level after about 30-40 years. After 20-30 years of sugarcane, organic C content had declined to about 33 g kg^-1 for the Hutton soil and 17 g kg^-1 for the Glenrosa soil. The higher organic matter content maintained at the Hutton site was attributed mainly to clay protection of organic matter since the clay content of the Hutton soil was 62% compared to 18% for the Glenrosa soil. For the same reason, aggregate stability was also generally higher in the Hutton than Glenrosa soil. The loss of soil organic matter under sugarcane resulted in a concomitant decline in soil microbial biomass C, percentage of organic C present as microbial biomass, basal respiration, fluorescein diacetate hydrolytic activity and aggregate stability. At the Glenrosa site, the natural '¹³C abundance in soils was used to calculate the loss of forest-derived, native soil C and the concomitant input of sugarcane-derived C. Sugarcane-derived organic C increased over time until it accounted for about 61% of organic C in the surface 10 cm in soils that had been under sugarcane for greater than 50 years. It was concluded that sugarcane production can cause a large decline in soil organic matter content and that practices such as green cane harvesting, zero tillage and use of green crops should be promoted to help ameliorate the problem.     

Soil organic matter,microbial properties,and aggregate stability under annual and perennial pastures

The use of annually sown pastures to provide winter forage is common in dairy farming in many regions of the world. Loss of organic matter and soil structural stability due to annual tillage under this management may be contributing to soil degradation. The comparative effects of annual ryegrass pastures (conventionally tilled and resown each year), permanent kikuyu pastures and undisturbed native vegetation on soil organic matter content, microbial size and activity, and aggregate stability were investigated on commercial dairy farms in the Tsitsikamma region of the Eastern Cape, South Africa. In comparison with soils under sparse, native grassy vegetation, those under both annual ryegrass and permanent kikuyu pasture had higher soil organic matter content on the very sandy soils of the eastern end of the region. By contrast, in the higher rainfall, western side, where the native vegetation was coastal forest, there was a loss of organic matter under both types of pasture. Nonetheless, soil organic C, K₂SO₄-extractable C, microbial biomass C, basal respiration, arginine ammonification and fluorescein diacetate hydrolysis rates and aggregate stability were less under annual than permanent pastures at all the sites. These results reflect the degrading effect of annual tillage on soil organic matter and the positive effect of grazed permanent pasture on soil microbial activity and aggregation. Soil organic C, microbial biomass C, K₂SO₄-extractable C, basal respiration and aggregate stability were significantly correlated with each other. The metabolic quotient and percentage of organic C present as microbial biomass C were generally poorly correlated with other measured properties but negatively correlated with one another. It was concluded that annual pasture involving conventional tillage results in a substantial loss of soil organic matter, soil microbial activity and soil physical condition under dairy pastures and that a system that avoids tillage needs to be developed.     

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.     

Losses of carbon and nitrogen with prolonged arable cropping from sandy soils of the South African Highveld

A knowledge of the kinetics of organic matter transformations in arable soils is important for managing them sustainably. Our aim in this study was to elucidate the effects of cropping period on pools of C and N in coarse‐textured savanna soils of the South African Highveld. Composite samples were taken from the top 20 cm of soils (Plinthustalfs) that have been cropped for lengths of time varying from 0 to 98 years in each of three different agro‐ecosystems in the Free State Province of South Africa. Thereafter, soil organic C and N concentrations were determined in the bulk soil (< 2 mm) as well as in the clay (< 2 μm), silt (2–20 μm), fine sand (20–250 μm), and coarse sand (250–2000 μm) separates. Long‐term cultivation of native grassland reduced soil C and N concentrations by 65 and 55%, respectively. Losses of soil organic matter occurred from all particle‐size separates, although rate loss constants increased as particle size increased. The concentrations of organic C reached equilibrium after 34 years for the bulk soil and after 55 years for clay‐size separates. Nevertheless, organic matter attached to silt continued to be lost as the cropping continued, probably due to wind erosion. Changes in soil properties thereby continued even after almost 100 years of cultivation.     

耕作对土壤有机物和土壤团聚体稳定性的影响

Agricultural sustainability relates directly to maintaining or enhancing soil quality. Soil quality studies in Canada during the 1980‘s showed that loss of soil organic matter (SOM) and soil aggregate stability was standard features of non-sustainable land management in agroecosystems. In this study total soil organic carbon (SOC), particulate organic matter (POM), POM-C as a percentage of total SOC, and aggregate stability were determined for three cultivated fields and three adjacent grassland fields to assess the impact of conventional agricultural management on soil quality. POM was investigated using solid-state ^13C nuclear magnetic resonance (NMR) to determine any qualitative differences that may be attributed to cultivation. Results show a highly significant loss in total SOC, POM and aggregate stability in the cultivated fields as compared to the grassland fields and a significant loss of POM-C as a percentage of total SOC.Integrated results of the NMR spectra of the POM show a loss in carbohydrate-C and an increase in aromatic-C in the cultivated fields, which translates to a loss of biological lability in the organic matter. Conventional cultivation decreased the quantity and quality of SOM and caused a loss in aggregate stability resulting in an overall decline in soil quality.     

Soil organic matter content and chemical composition under two rotation management systems in a Mediterranean climate

The crop rotation system in organic farming is a determinant factor to accumulate and preserve soil organic matter (SOM), and in depth knowledge on its effects is still lacking. Tillage intensity in particular is crucial to maintain soil aggregates and protect SOM from degradation. The evolution of SOM was tested in two adjacent fields under two different rotation cropping systems (low-intensity tillage and high-intensity tillage), and the effect of a further cultivation of legume in both fields was evaluated using ¹³carbon (C)-nuclear magnetic resonance (NMR) and elemental analysis of samples isolated through combined aggregate size and density fractionation. The two adjacent fields had been managed using the following organic farming methods for 13 seasons since 1998: i) alfalfa-based, with nitrogen (N) enrichment and low-frequency tillage with alfalfa (Medicago sativa) (9 seasons), winter wheat (Triticum durum) (3 seasons), and broad bean (Vicia faba) (1 season) and ii) cereal-based, with N depletion and annual tillage with barley (Hordeum vulgare) (7 seasons), sunflower (Helianthus annuus) (2 seasons), broad bean (Vicia faba) (3 seasons), and bare fallow (1 season). Soil sampling was carried out at the end of the 13-year rotation (T0, November 2011) and after winter wheat and chickpea cultivation in both fields over two subsequent years (T1, July 2013). Bulk organic C was significantly higher in the alfalfa-based system than in the cereal-based system at both T0 and T1, with SOM occluded in soil aggregates and associated with mineral particles. In terms of the macroaggregates heavy fraction at T0, the alfalfa-based field contained twice the organic C of that in the cereal-based field, as well as three times the organic C in the occluded particulate organic matter (POM). The occluded POM (oPOM) had a lower aryl/O-alkyl C ratio in the alfalfa-based system than in the cereal-based system, suggesting that oPOM undergoes a lower degree of decomposition during low-intensity management. The aryl/O-alkyl C ratios of the macro-and microaggregate oPOM decreased from T0 to T1 in the cereal-based system, suggesting increased protection of these fractions by soil aggregates. Thus, including legumes in crop rotation appears to positively affect the accumulation of SOM associated with mineral particles and within soil aggregates.