Soil structure associated with cover crops and grass leys in degraded lowland soils of Delta
Creators: name: Hermawan, Bandi
Publisher: name: University of British Columbia
Year: 2009

description: Rates of change of surface soil structure under different short-term cover crops and grass leys were studied on lowland soils in the Fraser River delta, British Columbia, Canada. The impact of cropping systems on soil structure was assessed under three environments: (i) short term (&lt;1 -yr) effects from overwinter cover crops, (ii) short term (&lt;3 -yr) effects associated with a grass ley reclamation program, and (iii) long term effects associated with varying cultivated crop and grass rotations. In the overwinter study, fall planted cover cropping treatments, including bare control, winter-killed spring barley, fall rye, and annual ryegrass (for 1993-1994 only), were compared at separate sites in 1992- 1993 and 1993-1994 for their effects on soil physical properties. At the reclamation study, two cropping practices - spring-sown barley underseeded with clover for winter cover cropping and grass ley - were established in a subsurface drained site and an adjacent poorly drained (no subsurface drainage) site. Measurements in the crop rotation study were conducted on five adjacent fields with different cropping histories. Aggregate stability, bulk density and aeration porosity did not differ among overwinter cover cropping practices at the 1992-1993 site. A lack of differences in this study was probably due to low above-ground crop biomass when compared to the 1993- 1994 winter cover cropping site. In contrast, overwinter cover crops in the 1993-1994 study had higher aggregate stability, aeration porosity and water infiltration, and lower bulk density and penetration resistance when compared to bare soil. Improved aggregate stability with cover crops in the 1993-1994 study was related to increased organic carbon in the soil, while increased aeration porosity and water infiltration were strongly correlated with higher earthworm populations under cover crops. When the 1993-1994 field was cultivated in spring, winter cropped soils had better structure than did soils left bare over winter. Overall, the greatest structural improvements were observed under annual ryegrass, followed by fall rye and winter-killed spring barley. After three years of grass ley program at the Reclamation Site, grass consistently had higher surface aggregate stability when compared to a combination of summer cash plus winter clover cover cropping. Improved aggregate stability with grass was significantly correlated with increased soil organic carbon content. Compared to the initial values measured in spring 1991, grass increased aggregate stability, expressed as mean weight diameter, by 0.4 to 0.7 mm and increased organic carbon content by 5 g kg⁻¹. Water infiltration under grass also increased following improved aggregate stability. The effects of cropping regimes on other soil structural characteristics varied with subsurface drainage conditions. Integration of cash and clover cover cropping maintained low bulk density and penetration resistance and high aeration porosity in the spring on the field with subsurface drainage. In the poorly drained field, however, bulk density was lower and aeration porosity was higher in the grass ley than in the cash-clover cover crop regime. Soil profiles also dried more quickly in winter and early spring under grass ley when subsurface drainage was not available. Earthworms migrated out into the field from their source at the ditch verges at a rate of about 15 m yr ⁻¹ under all cropping and drainage regimes. The period that land has been under grass or cultivated crops can be used to predict changes in selected properties of the soil at any given time. In the crop rotation study, the period of cultivated crops since grass was strongly correlated with organic carbon level in the soil. Organic carbon decreased significantly in the order of 2-yr grass (0-yr cultivation) = 1-yr cultivation &gt; 5-yr cultivation = 8-yr cultivation &gt; 11-yr cultivation. When soil water content at sampling was taken into consideration, previous cropping history was correlated with aggregate stability. Most structural characteristics observed in overwinter cover cropping and grass ley studies were significantly influenced by the time of sampling. Aggregate stability decreased from fall to spring, then increased again over summer and fall. The seasonal variation in aggregate stability was attributed primarily to soil water content at the time of sampling. Aeration porosity and water infiltration also decreased, while bulk density and penetration resistance increased during fall-to-spring months. The seasonal variation in structural characteristics was much more pronounced in the least stable soils, such as bare and continued cash-winter cover cropping conditions, than in the more stable cover cropped and grass ley soils.