INCREASED PROPORTION OF ACTIVE SOIL-N IN BRETON LOAM UNDER CROPPING SYSTEMS WITH FORAGES AND GREEN MANURES

Total soil N and N mineralization rate partially characterize the infl uence of various cropping systems on the growth of sequent crops in a rotation. The objectives of this study were to (1) quantify the relati onship among cropping system. total N and mineralizable N; and (2) com pare amount of N mineralized under controlled laboratory conditions wi th plant N uptake under greenhouse conditions. Three cropping systems that have been in operation between 9 and 60 yr on a Gray Luvisol (Bre ton loam) were selected. They included: (1) an agroecological (AER) 8- yr rotation involving fababeans as green manure (AERI sampled after th e first fababean crop and AER2 sampled after 3 yr of continuous forage ); (2) continuous grain system (CG), with fertilizer N at 90 kg h-1 yr -1; initiated in 1980 and considered established in 1981; (3) a classi cial Breton rotation (CBR) involving a long-term (ca. 1930) 5-yr rotat ion with forages and cereals and no return of crop residues (CBR1 fert ilized with P-K-S and CBR2 unfertilized). We caution that not all phas es of each rotation were sampled; our conclusions pertain to N-mineral ization potential in soil samples immediately preceding barley as sequ ent crop in each rotation. The rate of N mineralization declined with time, but it remained greater than zero after 20 wk of incubation in a ll soils. Mineral-N accumulation at 20 wk followed the order AER1 > AE R2 > > CBR1 > CBR2 = CG. Mineralizable soil N, following one cycle of the AER rotation, was almost double that following 60 yr of the CBR ro tation. Data for mineral-N accumulation under laboratory conditions we re described best by a single-component exponential model. Legume-base d rotations were associated with increased total soil N and a greater proportional increase in active N than in total soil N. Active N was l east in soil under the CG system. The incubation-extraction procedure resulted in higher estimates of mineralizable N than did the plant-upt ake method; however, the ranking of N-supplying power of soils was the same.