Increased proportion of active soil N in Breton loam under cropping systems with forages and green manures

Abstract

Total soil N and N mineralization rate partially characterize the influence of various cropping systems on the growth of sequent crops in a rotation. The objectives of this study were to (1) quantify the relationship among cropping system, total N and mineralizable N; and (2) compare amount of N mineralized under controlled laboratory conditions with plant N uptake under greenhouse conditions. Three cropping systems that have been in operation between 9 and 60 yr on a Gray Luvisol (Breton loam) were selected. They included: (1) an agro-ecological (AER) 8-yr rotation involving fababeans as green manure (AER1 sampled after the first fababean crop and AER2 sampled after 3 yr of continuous forage); (2) continuous grain system (CG), with fertilizer N at 90 kg ha−1 yr−1; initiated in 1980 and considered established in 1981; (3) a classicial Breton rotation (CBR) involving a long-term (ca. 1930) 5-yr rotation with forages and cereals and no return of crop residues (CBR1 fertilized with P-K-S and CBR2 unfertilized). We caution that not all phases of each rotation were sampled; our conclusions pertain to N-mineralization potential in soil samples immediately preceding barley as sequent crop in each rotation. The rate of N mineralization declined with time, but it remained greater than zero after 20 wk of incubation in all soils. Mineral-N accumulation at 20 wk followed the order AER1 > AER2 > > CBR1 > CBR2 = CG. Mineralizable soil N, following one cycle of the AER rotation, was almost double that following 60 yr of the CBR rotation. Data for mineral-N accumulation under laboratory conditions were described best by a single-component exponential model. Legume-based rotations were associated with increased total soil N and a greater proportional increase in active N than in total soil N. Active N was least in soil under the CG system. The incubation-extraction procedure resulted in higher estimates of mineralizable N than did the plant-uptake method; however, the ranking of N-supplying power of soils was the same. Key words: Cropping systems, Gray Luvisol, N mineralization, soil quality, Typic Cryoboralf