Abstract
Nitrogen (N) dynamics during changes in land use patterns in tropical forests may profoundly affect fine root dynamics and nutrient cycling processes. Variations in fine root biomass and soil N dynamics were assessed in developing stands of increasing ages following shifting agriculture in Mizoram, Northeast India, and comparisons were made with a natural forest stand. Concentrations of soil available N (NH4-N and NO3-N) and the proportion of NH4-N in total available N increased with stand age. The N-mineralization rate also increased with stand age whilst the proportion of nitrification relative to ammonification declined during succession. Fine root biomass and N-mineralization increased, and available N decreased during the monsoon season while this pattern was reversed during the winter season. A greater proportion of fine roots were <0.5 mm diameter in the younger sites, and turnover of fine roots was more rapid in the developing stands compared to the natural forest. Fine root biomass was correlated positively with N-mineralization rate and soil water content. Thus, it can be concluded that the fine root growth was aided by rapid N-mineralization, and both fine root growth and N-mineralization increase as stands redevelop following shifting cultivation disturbance.
Highlights
Landscape transformations are rapidly occurring in tropical regions due to anthropogenic activities that alter ecosystem structure and functioning including nutrient cycling and other below-ground processes [1,2,3]
Total fine root mass in this study was comparable to that found in other reports from tropical forests of India [3,33,34,35] and increased with stand age, in agreement with other studies in boreal [36]
The variation in fine root biomass among the developing stands and natural forest can be attributed to differing soil nutrient and moisture content, species composition and stand above-ground biomass and tree density [18]
Summary
Landscape transformations are rapidly occurring in tropical regions due to anthropogenic activities that alter ecosystem structure and functioning including nutrient cycling and other below-ground processes [1,2,3]. Slash-and-burn and the subsequent shifting agriculture is a common land-use in many parts of the tropics [4], including in Mizoram in Northeast India [5,6]. During the shifting cultivation process, forests are cut down and burned, and crops are grown for one or a few years depending on the soil nutrient status, after which the land is left fallow and allowed to restore natural vegetation and soil nutrients. The sustainability of this land-use depends upon the successful replenishment of nutrient stocks during the fallow phase. Biomass burning further leads to particulate and volatilization losses of both nitrogen (N) and phosphorus (P), the extent of this loss appears to depend upon soil type [7,8]
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