Organic amendment and rotation crop effects on the recovery of soil organic matter and aggregation in potato cropping systems

Soil structural degradation is common in intensively cultivated ecosystems due to the depletion of soil organic matter (SOM). We investigated the mechanisms by which different frequencies of organic amendment application and rotation crops restore C, N, and aggregation in gravelly loam soils used for potato production. A single amendment application [FIRST; 22 Mg ha(-1) compost and 45 Mg ha(-1) beef cattle (Bos taurus L.) manure] did not affect total C in 1996 and increased it by 28% in 1997 relative to unamended plots (NONE); light fraction (LF) C accounted for 56% of this increase. Plots in which amendment was suspended for 1 yr (SASP) following 4 or 5 yr of annual application had more total C in 1996 (28%) and 1997 (46%) relative to NONE. A green manure crop consisting of oat (Avena sativa 'Porter'), pea (Pisum sativum 'Trapper'), and hairy vetch (Vicia villosa Roth) grown in 2-yr rotation with potatoes (Solanum tuberosum L.) increased soil C in 1997 (25.9 vs. 23.9 g kg(-1)), LF properties in 1996 and 1997, and water soluble carbohydrates (WSC) on several sample dates relative to an oat rotation crop. Large aggregate (2-6.5 mm) stability in 1996 and 1997 and medium aggregate (1-2 mm) stability in 1997 were increased by FIRST relative to NONE. Total soil C was more strongly related to medium (r = 0.65 in 1997) and large (r = 0.51 in 1997) aggregate stability than LF or water soluble carbohydrate fractions. Compost and manure influences occurred rapidly and were persistent, demonstrating that annual applications are not necessary to reverse soil degradation.