Potencial de mineralização anaeróbica do nitrogênio em solos de várzea do Rio Grande do Sul

Abstract

The soil organic nitrogen is the main source of N to crops, which is released from soil organic matter (SOM) decomposition by microrganisms. The N fertilization is based on SOM levels, however, the SOM decomposition and mineralization in flooded soils has a different dynamic compared to well-drained soils, what makes difficult the estimate of N fertilizer doses to rice production. Therefore, the understanding of anaerobic SOM decomposition dynamic and N behavior can improve the N fertilizer recommendations. The objective of this work were: (a) to evaluate the anaerobic N mineralization potential in different soils from Rio Grande do Sul state in greenhouse and in vitro and its relations with soil characteristics, and (b) to fit mathematical models to describe the N mineralization potential and its relations with N availability to rice. Fifteen soils samples from superficial layer (0 to 0,2m) of lowland soils from RS state were collected, air-dried, and passed through a 2 mm sieve. To standardize the soil pH, limestone was mixed to reach pH 5,5, based on SMP index of each soil. After a 35-days period, the soils were set in pots with four replications in an entirely randomized experimental design. The rice grew in a 60-day period in two successive cultivations in greenhouse and the shoot was taken to determine absorbed N. N anaerobic incubation was set for 24 weeks, where the flooded solution was colleted and amount of N was measured. On these results, five mathematical models were fitted to estimate N mineralization potential and N mineralization rates. It was evaluated the correlations between absorbed N and soils characteristics, mineralized N, or model parameters. The soil organic carbon and clay content were high correlated, respectively, with absorbed N in first and second cultivations. The accumulated mineralized N presented high correlation with soil organic carbon and absorbed N in first cultivation. The better model to estimate the N potentially mineralizable (N0) and mineralization rates (k) was of the Jones (1984). The N0 and k estimated by Stanford & Smith (1972) model presented high correlations, respectively, with absorbed N in the second cultivation and clay. The N2 estimated by Jones (1984) model was correlated with absorbed N in the second cultivation.