MINERALIZACIÓN BRUTA DE NITRÓGENO BAJO LABRANZA CONVENCIONAL, SIEMBRA DIRECTA Y PASTURA.

The intensification of conventional agriculture in the southeastern Buenos Aires province (Argentine) produced soil fertility degradation. No-tillage (SD) agriculture and the inclusion of pastures (PP) in crop rotations, represent management alternatives to reduce conventional tillage (LC) effects. A lab experiment was carried out with the aims of: a) to study nitrogen mineralization (TMB), nitrification (TNB) and consumption (TCB) gross rates in a Mollisol under LC, SD and PP; and b) to quantify the potentially mineralizable nitrogen through anaerobic incubation (NAN) and its relation with mineralization rates. The studied soil was a Typic Argiudoll - Petrocalcic Paleudoll complex which had been under conventional tillage for 17 yr until 1994 when it was converted to three different soil management systems: continuous PP and continuous cropping under SD, and under LC. Gross processes were estimated through the 15N dilution technique, adding 15N-labeled (NH4),SO4 (10% 15N at. exe.) four times during an 35-d incubation period and NAN was determined through short anaerobic incubations. Gross N mineralization rates ranged between 0.17 and 1.36 mg N kg-1 day-1. The soil under PP treatment presented higher TMB than cropping managements. Gross consumption rates ranged between 0.47 and 3.21 mg N kg-1 day-1, being nitrification the most important consumption process for all three management systems (80%). Cumulative mineralized N (P= 28.6 mg kg-1, SD= 19.7 mg kg-1 and LC= 10.8 mg kg-1) was different among management systems, whereas cumulative consumed N were not (P= 72.6 mg kg-1, SD= 56.9 mg kg-1 and LC= 74.1 mg kg-1). The soil under PP presented highest NAN values (93.4 mg kg-1) that was twice as much as soil under LC (41.4 mg kg-1) or SD (46.6 mg kg-1). Lineal correlation coefficients between TMB and NAN were statistically significant for all the measurement times and ranging between 0.47 and 0.97. After five years from management installation, PP produced an increase in TMB relative to the cropping managements, whereas the same period under SD did not improve the TMB relative to LC. Gross consumption rates did not present differences among management systems and neither PP nor SD improved TNB respect to LC. [ABSTRACT FROM AUTHOR]