Effects of soil texture, carbon input rates, and litter quality on free organic matter and nitrogen mineralization in Chilean rain forest and agricultural soils

Nitrogen (N) mineralization in soil depends on carbon (C) input of plant materials, site environmental conditions, and soil texture. Empirical correlates of N mineralization can be difficult to interpret, however, because of interactions among these factors. A multiple regression model relating N mineralization to C input rates, litter quality, and soil texture in Chilean temperate rain forest soils was developed. Nitrogen mineralization rates ranged from 3.0 to 5.7 mg kg-1 soil d-1 depending on C input rate and soil texture but were not influenced by litter quality. We compared C storage of both forest and long-term-cropped soils with the protective capacity (i.e., the expected maximum C pool associated with the clay and silt fractions). Soil organic C associated with the fine fraction of forest soils was significantly greater than the calculated protective capacity, with clay-rich soils averaging 141% more C than this limit and coarser soils having 56% more than predicted. In contrast, C content of cropped soils was well below the calculated protective capacity, averaging ∼32-60% less than this limit, showing the potential of these soils for sequestering C. The results were consistent with the finding that N mineralization was positively correlated with the amount of free organic matter associated with the sand-size fraction in forest soils. The study illustrates that (i) the capacity of soils to preserve soil organic C in clay- and silt-sized particles was greater than that of agricultural soils and (ii) in highly saturated soils, the N mineralization is a function of the quantity of organic-matter input, which in turn accumulates as free organic C in the sand-size fraction.