A combined BaPS-13C stable isotope technique to study the interaction between C and N turnover in alkaline agricultural soils of the North China Plain

In the intensively managed double-cropping production system of the North China Plain, the excessive use of  nitrogen (N) fertilizer has resulted in adverse environmental impacts such as leaching of nitrate to shallow groundwater or gaseous losses of the greenhouse gas N₂O. An understanding of N cycling in soil is essential for deriving sustainable fertilization strategies. Nitrogen transformations in soil are closely linked to the carbon (C) cycle. All heterotrophic decomposing micro organism simultaneously assimilate C and N during decomposing plant residues or soil organic matter. An understanding of this linkage is important, for example, for assessing the feedback of a changed N fertilization practice on the soil organic matter pool. To study and quantify the C and N fluxes in soil, we need a set of reliable and accurate methods. During the last decade a novel method, the Barometric Process Separation, has been used to measure gross nitrification rates in soil. Recently, it has been shown that the use of the BaPS method becomes problematic at soil pH>6. At pH values above 6 the BaPS calculation is strongly affected by the CO_2,aq term, i.e. the dissolution of gaseous CO₂ during incubation. So far, no methods are available to accurately quantifying this term. In our study, we aim at developing a novel combined Barometric Process Separation (BaPS)-¹³C stable isotope technique, which allows an accurate quantification of the CO_2,aq term. In parallel, we will study to which extent the incorporation of plant residues of different quality immobilises surplus soil nitrate and its potential to reduce nitrate leaching in soils with a nitrate-dominated mineral N pool. Moreover, we will study the mid- and short term interaction of C and N turnover at the process-level to get a better understanding on the feedback mechanism between both matter cycles.