FraÃÃes de carbono e de nitrogÃnio em uma microbacia com diferentes usos do solo.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

Productivity in subsistence or low input agricultural systems is largely dependent on nutrient supply from soil organic matter mineralization. For this reason, quantity and quality of soil organic matter are fundamental variables that control sustainability in subsistence agricultural systems. The hypothesis of this work was that soil organic matter quantity and quality are significantly affected by changes in land use and by relief. To study the interrelationships among those variables, soil samples (n=362) were collected within the Vaca Brava watershed, located in the municipalities of Remigio and Areia (PB). Sampling was stratified by soil use (subsistence agriculture, Mimosa caesalpiniaefolia reforestation, bush-fallow, napiergrass, grassland, and native forest) and positions in the relief (summit, shoulder, backslope, footslope, and alluvium). Samples were analyzed for total C (Ct), total N (Nt), oxidizable C (Cox), amino-sugar N (Namino), and particle-size distribution. Five textural classes were also included in the analysis of variance. No interactions were observed among land use, relief and texture in the Ct, Nt and Cox concentrations or Cox/Ct and Namino/Nt ratios. The only exception was the interaction of land use x texture upon Namino concentrations. Average concentrations of Ct, Cox and Nt were greater (p<0,05) under native forest (27,4, 2,34 and 1,67 g kg-1 soil, respectively), intermediate (p<0,05) under bush fallow and reforestation (16,0, 1,29, 1,02 g kg-1 soil, respectively) and smaller under napiergrass, grassland and subsistence agriculture (11,4, 0,94, 0,71 g kg-1 soil, respectively). These concentrations probably represent three distinct levels of land use intensity. Namino concentrations also decreased in the same order but statistical significance were attained or not depending on textural classes. Namino/Nt and Cox/Ct ratios (showed as percentages) were considered as indexes of soil organic matter quality, since they indicate the proportion of labile Ct and Nt. The Namino/Nt ratios were greater (p<0,05) under native forest (7,11%), reforestation (7,56%) and grassland (7,16%) than under subsistence crops. The Cox/Ct showed a different trend, having greater values (p<0,05) under reforestation (6,89%) and napiergrass (6,61%) and lower (p<0,05) under grassland (6,01%). To increase sensitivity in the analysis of data related to the more intense land uses these were re-analyzed separately, excluding native forest, reforestation and bushfallow data. Concentrations of Ct, Nt, Namino and Namino/Nt were greater in grasslands than under subsistence agriculture (p<0,05), while areas under napiergrass had intermediate Ct and Nt concentrations that did not differ from the other two land uses. Concentrations of Cox were similar under grasslands, subsistence agriculture and napiergrass, indicating that it is less sensitive than Namino to land use change. In relation to texture (n=362), the greater concentrations of Ct (26,7 g kg-1 soil), Nt (2,20 g kg-1 soil), and Cox (1,44 g kg-1 soil), were found in clay and clay-sand textures (p<0,05); the smaller concentration occurred in the sandy loam textures (p<0,05) (7,94, 0,67 and 0,54 g kg-1 soil, respectively). The Cox/Ct ratio increased with increasing finer textures while Namino/Nt was not affected by textural changes. The effect of position in the relief was analyzed for the complete data set and it influenced concentrations of Ct, Cox, Nt and Namino (p<0,05). The greater concentrations (p<0,05) of Ct, Cox, Nt and Namino were found at the summit, backslope and alluvium while the smaller concentration were found at the footslope. Overall, Cox and Namino contents followed changes in Ct and Nt contents. For this reason, the ratios Cox/Ct and Namino/Nt varied with land use change within a narrow range that, although statistically significant, left doubts about their potential use as a diagnostic tool for soil organic matter changes in quality

ASSUNTO(S)

microbacia fraÃÃes carbono solo engenharia nuclear nitrogenio

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