Pedotransfer functions for estimating total soil nitrogen up to the global scale

Abstract

Realistic model representation of the dynamics of terrestrial ecosystems, including all relevant biophysical and biogeochemical interactions, is a prerequisite to accurate predictions of climate change. However, the current generation of models requires improvement. In particular, it is important to know the nitrogen status of soil for modelling global vegetation and the emission of pollutants such as nitrous oxide. For this reason, we collated several major datasets that record total nitrogen (TN) and other key properties of soils. From these data, we derived pedotransfer functions (PTFs) to predict TN of soils from other soil and site properties using stepwise multiple regressions. The most important predictor was found to be soil organic carbon (SOC). Other helpful soil and site properties for predicting TN included distance from the equator, soil texture class, soil group C:N ratio, pH, % clay and whether the soil contains more or less than 10% SOC. Several different PTFs are presented, giving the user a choice, depending on which soil and site properties are available. In all, we calculated that over 9000 observations were available, although not all datasets contained all the site properties listed above. The most useful PTF was found to be: TN = 0.0232 + 0.0250*C− 0.000534*D + 0.521*G + a(T), for C < 10%, and TN = 0.269 + 0.0449*C− 0.00575*D− 0.157*G + b(T), for C≥10%, where C is % SOC, D is the degrees from the equator, G is a soil group factor defined as (C/Median C:N ratio of soil group), T is a factor based on texture classes (1, 2 or 3), depending on clay and sand content and a takes the value 0.0, 0.0228 or 0.0296 when T is 1, 2 or 3, respectively, and similarly b takes one of the values 0.0, 0.328 or 0.440, respectively. The PTFs were validated against data not used in their derivation, which were held back for this specific purpose. On the basis of the PTF above and the pre‐existing digital soil map of the world, we publish a map of the global distribution of TN. It has been suggested that additional CO2 may be emitted from soils as a result of global warming, exacerbating warming still further. We calculate that the nitrous oxide formed as a result of the nitrogen that also mineralized could contribute a further 30% of additional warming.