Soil organic carbon as a key predictor of N in forest soils of Brazil
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Despite the ancillary knowledge that soil N is chiefly retained as soil organic matter, little is known about how it is affected by other soil and environmental factors, especially in the tropics. In this study, we performed a comprehensive survey of soils under native vegetation in Minas Gerais, Brazil, aiming to (a) measure soil Kjeldahl-N concentrations to a 1-m depth, (b) identify the main affecting factors of soil N retention, and (c) predict N through soil profile based on organic C (SOC) and its main conditioning factors.
Materials and methods
Soils under 36 fragments of native forest and savanna were sampled at five depths (0–10, 10–20, 20–40, 40–60, and 60–100 cm) and characterized by physical and chemical analyses, including total N determined by the micro-Kjeldahl method. Single and multivariate regressions were used to predict N concentrations based on soil properties and climatic factors.
Results and discussion
The average N concentrations ranged between 0.12 and 7.54 g kg−1, decreasing with depth, and can be predicted using SOC concentrations (R 2 = 0.86). Multivariate regressions using more input data, namely texture, cation exchange capacity (CEC), and altitude increased slightly R 2 values (0.68–0.90) for separate soil depths, but not for the whole dataset (R 2 = 0.85).
We demonstrated that N can be adequately predicted based on SOC concentrations, for any depth and forest type. The implications of the stable SOC/N relation and their coupled cycles and the environmental factors affecting N retention in Brazilian weathered soils are further discussed.
KeywordsC and N soil cycles Cerrado Soil texture Pedotransfer functions Tropical forest Weathered Brazilian soils
This research was funded by the CNPq (process 308592/2011-5), Fapemig (process CAG - APQ 00291-11), and Cemig. The authors are grateful to the staff of the Department of Forest Science (Lemaf), Federal University of Lavras, involved in field sampling, processing, preparation and some of laboratory analysis of soil samples. We thank student Henrique J. G. M. Maluf (Federal University of Lavras) for the help with the principal component analysis.
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