Soil quality as affected by agroforestry and grass buffers in grazed pasture and row crop systems

Establishment of agroforestry and grass buffers within agroecosystems is believed to improve soil quality. Soil enzyme activities and water stable aggregates have been identified as sensitive soil quality indicators to evaluate early responses to soil management. However, only a few studies compared these parameters among buffers, grazing pastures, and row crop systems. The objective of this study was to compare the activities of selected enzymes (beta-glucosidase and beta-glucosaminidase, fluorescein diacetate (FDA) hydrolase, dehydrogenase), water stable aggregates (WSA), soil organic carbon (SOC), total nitrogen (TN), and bulk density as soil quality parameters among four management treatments: grazed pasture (GP), agroforestry buffer (AgB), grass buffer (GB) and row crop (RC). Two soil depths (0-10 and 10-20 cm) were analyzed in all treatments for two consecutive years, 2009 and 2010. The WSA was determined by wet sieving method while enzyme activities were colorimetrically quantified using a spectrophotometer in laboratory assays. Soil organic carbon, TN, and bulk density were also determined by standard procedures. Most of the soil quality indicators were significantly greater in perennial vegetation treatments compared to row crop. The dehydrogenase activity in the GP treatment was 323.8 mug TPF g -1 dry soil while it was 174 mug TPF g-1 dry soil in RC treatment. Similarly, the GB treatment showed an activity of 811.4 mug fluorescein g-1 dry soil in 2010 for FDA enzyme. Although there were numerical variations, the trends in response of quality parameters were consistent between years. The beta-glucosaminidase activity increased slightly from 155.6 to 177.0 mug PNP g-1 dry soil while beta-glucosidase activity decreased slightly from 248.0 to 236.6 mug PNP g-1 dry soil in GB treatment during two years. Water stable aggregates increased from 17.8 to 31.4% in row crop while all other treatments had similar values during the two-year study. Surface soil revealed greater enzyme activities and WSA than the sub-surface soil. The treatment by depth interaction was significant (P<0.05) for beta-glucosidase and beta-glucosaminidase enzymes in 2009 while the interaction was significant (P<0.05) for dehydrogenase and beta-glucosaminidase in 2010. Soil enzyme activities were significantly correlated with soil organic carbon content (r=0.78 to 0.94; P<0.0001). The nature of enzyme activities observed in this study support the hypothesis that perennial vegetation provides favorable conditions for greater enzyme activities and microbial diversity compared with soils under row crop management. Hence the RC treatment can be assigned a soil quality index of 0.43, while the perennial vegetation treatments could attain an index of 0.63-0.67 with respect to a reference soil according to the arithmetic method. Assessing changes in selected enzyme activities appears to be a useful tool to determine soil degradation when reference values for similar systems are available. Implications can be made that perennial vegetation enhances organic matter accumulation in the soil, has minimum disturbance to the soil and will improve soil quality indicators.