Nitrogen mineralisation along a pH gradient of a silty loam UK soil

Most investigations into the effects of changing soil pH on microbial activity use, from necessity, soils taken from different sites so that soil physical and chemical properties are confounded. Studies along continuous gradients of soil pH within a single soil type are rare, simply because so few exist, in UK or even worldwide. Here we report measurements of mineralisation of native organic matter and added arginine along a continuous soil pH gradient (range about pH 3.7–8.3) of a UK silty clay loam soil (Chromic Luvisol or Typic Paleudalf). The soil has been maintained under constant management for more than 100 years, with winter wheat sown annually. The soil NH₄⁺-N concentration was maximal at the lowest pH (pH 3.7), declining exponentially until pH 5.5 and remaining negligible thereafter. However, unexpectedly, soil NO₃^?-N concentration was also maximal at pH 3.7 and was significantly negatively correlated with increasing pH thereafter. To investigate these unexpected NO₃^?-N results, arginine was added as a labile source of organic N and its extent of ammonification and nitrification measured at soil pHs 3.79, 4.42, 6.08 and 7.82. While arginine ammonification was apparently greatest at pHs 3.79 and 4.42, similar to mineralisation of soil organic N, nitrification of this added N was greatest at soil pH 7.82 and least at pH 3.79, the reverse of the situation with soil organic N, but much more in line with what was expected. It was concluded that the decline in soil NO₃^?-N with increasing pH in the unamended soils was an artefact, caused by increasing plant uptake of NO₃^?-N as yield increased, rather than a true effect of low pH increasing nitrification of soil organic N. Our results differ from most previous studies, which showed poor correlations between soil pH and arginine mineralisation. This was attributed to our use of much longer incubation times (up to 50 days) than usually employed. Under our conditions, arginine was therefore shown to be a useful model for mineralisation of labile soil organic N.