The effect of long-term no-till crop rotation practices on the soil organic matter functional pools

[摘要] ENGLISH ABSTRACT: Total soil organic matter (SOM) and its different functional pools (fractions) are important attributes of the physical, chemical and biological quality of the soil and are seen as key factors in the evaluation of the sustainability of management practices. Until now, limited information was available regarding soil C accumulation and stabilization under conservation tillage managed soils in the Western Cape grain production regions of South Africa. Long-term field experiments investigating different crop and crop/pasture rotation systems under no-tillage were initiated in 2002 at the Tygerhoek Research Farm of the Western Cape Department of Agriculture, near Riviersonderend, Overberg, Western Cape, South Africa. The study site enabled us to compare the following five dryland cropping systems; permanent Lucerne (100% pasture), Medic-Medic-Wheat (MMW) (67% pasture; 33% crop), Medic-Medic-Wheat-Wheat (MMWW) (50% pasture, 50% crop) and two 100% cropping systems (continuous cropping) in different phase [Wheat-Barley-Canola-Wheat-Barley-Lupin (WBCWBL4 & WBCWBL1)]. The numbers '1 and '4 in rotation code refers to the first and fourth crop planted in the cropping system, respectively. The underlined crop in rotation code represents the crop that was on the field at time of sampling. Natural vegetated soil (non-cultivated area) acted as a reference for this study. In 2012, soil samples were taken at four depth increments; 0-5, 5-10, 10-20, 20-30 cm.The objectives of the study were to investigate the effect of long-term crop/pasture rotation systems on: i) the total soil organic carbon (SOC) storage under different cropping systems, ii) the SOC and N content in different functional pools (fractions); free particulate organic matter (fPOM) fraction (labile fraction), occluded particulate organic matter (oPOM) fraction (moderately stabile intra-aggregate C) and mineral-associated fraction (stabile fraction), (iii) the main C stabilizing mechanisms operative in these soils and (iv) the relationship between the extent of C sequestration and crop yields.After 11 years, the medic-wheat rotations had the highest total SOC contents (15.2-18.6 g kgˉ¹ in 0-30 cm depth, P ≤ 0.05), compared to the continuous cropping (13.3-14.1 g kgˉ¹ in 0-30 cm depth), permanent lucerne pasture (15 g kgˉ¹) or natural vegetated soil (13.2 g kgˉ¹). Higher belowground C inputs through roots and the lower extent of disturbance in the 0-10 cm depth are the main reasons for higher total C content in the wheat-medic systems compared to the other systems.The contribution of the fPOM fraction (labile C) to total C content in the cultivated treatments (6-9%) was lower than the natural vegetated soil (13%) in the 5-10 cm depth. The fPOM fraction is the most sensitive soil organic C and N pool to detect changes due to management practices, which include quantity and quality of OM inputs, extent of physical disturbance, and fertilization. The medic-wheat rotations had the highest C (1.37-1.74 g kgˉ¹ in 5-10 cm depth) and N (0.107-0.110 g kgˉ¹ in 5-10 cm depth) contents in the fPOM fraction of the cultivated treatments. Compared to the natural vegetated soil, the cultivated treatments had a lower C content in the oPOM fraction (moderately stabile fraction) and concomitantly a lower aggregate stability. On average, the oPOM fraction only contributed 0.4-2.4% to total C content at all sites. A significant positive correlation (R²= 0.77) was found between C occluded in aggregates (oPOM fraction) and aggregate stability with the highest aggregate stability found in the medic-wheat rotations of the cultivated treatments. The major part (85-93%) of the SOC was associated with the mineral fraction (stabile fraction) in the natural vegetated and agricultural soils. The MMWW treatment contained the highest C content (18.7 g kgˉ¹, 5-10 cm depth) in the mineral-associated fraction and the two continuous cropping systems the lowest (14.2-14.7 g kgˉ¹, 5-10 cm depth) of the cultivated treatments. A significant positive correlation was found between mineral-associated SOC fraction and clay (R² = 0.74) and Fe-oxide (R² = 0.57) content. This helps explain the large mineral SOC fraction found in these soils and is the dominant SOM stabilization mechanism operative in these shale-derived soils. The mineral-associated organic matter is probably predominantly sorbed to the clay minerals (illite, kaolinite and sesquioxides) via ligand exchange resulting in very strong organo-mineral associations. Physical protection via occlusion in aggregates is not a dominant C stabilizing mechanism in these soils. The C:N ratios of the fractions decreased in the order fPOM > oPOM > mineral with a C:N ratio below 10 in the mineral fraction indicative of humified organic matter.The MMW and MMWW treatments produced higher wheat yields in 2012 with a significant positive correlation found between total soil C and N, and yields obtained. In a higher quality soil, higher agronomic production is expected. Findings in this study enabled us to conclude that due to effect of cropping system and soil properties, the MMWW treatment had the highest total SOC content, which included highest labile C and N content and highest.

[发布机构] Stellenbosch University