Factors Controlling Soil Carbon and Nitrogen Storage in Puerto Rico and the US Virgin Islands

Abstract

Tropical soils represent a large and important carbon (C) reservoir and thus play an important role in the global C cycle. The accumulation and persistence of C in soils has implications for soil fertility and climate change. The amount of C stored in a soil can be affected by changes in land cover and human land use, although the direction and magnitude of the change is not consistent across studies, especially when comparing forests to pastures. Environmental factors, including soil type, soil texture, soil pH, precipitation, and temperature all have the potential to influence soil C storage, and may be important in determining how soil C stocks respond to land-use change. The goal of this study was to further explore the factors influencing regional patterns in soil C storage, with a focus on soil order and land cover. Soils were collected from 25 sites representing 19 soil series in three soil orders, and under two land cover types, pastures and forests, in Puerto Rico and the US Virgin Islands. Soils were analyzed at three depths, 0-100 cm, 0-30 cm, and 30-100 cm, for total carbon (TC), soil organic carbon (SOC), and total nitrogen (N). Soil order was an important predictor of soil TC, SOC, and soil N stocks for all depths. Mollisols consistently had the greatest stocks of TC, SOC and N, while Oxisols had the smallest stocks. Inceptisol stocks were intermediate, and the most variable, given the diversity of parent materials. Land cover was only a marginally significant predictor of SOC for 0-30 cm depth, at which forest soils contained 24 % more SOC than pasture sites. There was no significant interaction between soil order and land cover. Soil C:N ratios were greater in forest surface soils (0-5 cm) than in pasture soils indicating a difference in litter input chemistry or decomposition. Soil pH correlated positively with SOC and TC stocks, probably due to a number of calcium carbonate-rich Mollisol sites. Clay and fine silt content was positively yet weakly correlated with SOC stocks, suggesting that differences in soil mineralogy across the sites may be more important in predicting SOC stocks than texture alone. Mean annual temperature and mean annual precipitation were not correlated with SOC stocks in this study. These findings suggest that at a regional scale, both soil properties and land cover are important predictors of SOC stocks, and that patterns depend on the depth studied. Predictions of regional C stocks should account for the wide variability of soil types present in the tropics.