Soil Aggregation, Aggregate Carbon and Nitrogen, and Moisture Retention Induced by Conservation Tillage

We investigated the effects of 13 yr of plow tillage (CT), no‐tillage (NT), and ridge tillage (RT) on soil aggregation and moisture holding capacity under two cropping systems, corn ( Zea mays L.) alone and cotton ( Gossypium hirsutum L.) followed by corn at two depths. The experiment was conducted on an Hidalgo sandy clay loam (fine‐loamy, mixed, active, hyperthermic Typic Calciustoll). Few cropping system differences were found. Aggregation was significantly greater at the 0‐ to 5‐cm depth with NT and RT, especially in the >4750‐ and 500‐ to 212‐μm size classes, where aggregate C and N contents were as much as 60% and >100%, respectively, higher than in CT. At 10 to 15 cm, CT produced greater aggregation in all but the >4750‐μm size class but showed little enhancement of C and N retention compared with NT and RT. Mass‐weighted data revealed a more biphasic retention of C and N at the 0‐ to 5‐cm depth; more C and N were retained in the >4750‐ and 500‐ to 212‐μm size classes at 0 to 5 cm. Most C and N was detected in the >4750‐μm size fraction at the 10‐ to 15‐cm depth. Water holding capacity was significantly greater with NT and RT by >12% over CT management. The beneficial effects of conservation tillage are directly related to soil content and accumulation of C and N. In this hot climate, in which crop residues are rapidly oxidized, soil C and N accretion rates with conservation tillage are slow but demonstrable.