The effects of drying and rewetting cycles on carbon and nitrogen dynamics in soils of differing textures and organic matter contents

Many researchers have reported differences in soil C and N dynamics between soils of different textures and/or soil organic matter contents. However, it has proven difficult to determine the exact relationships and mechanisms between C and N dynamics and soil texture/SOM. There are few studies that consider how these soil physical and chemical conditions influence the effects of drying and rewetting on the mineralisation of C and N and the microbial transformations that follow. The objectives of this study were: 1) To determine the effects of repeated drying and rewetting cycles on C and N dynamics in soils of differing textural class and organic matter levels. 2) To use C & N mineralised at constant moisture contents to calculate mineralisation during dry/wet cycles for comparison with actual mineralisation. Two soil types with contrasting textures were chosen and 6 paddocks on each soil type were selected to produce an OM gradient for each soil. Three moisture treatments were chosen to simulate moist (field capacity at -0.01 MPa), moderately dry (120% of wilting point at -1.5 MPa) and very dry (80% of wilting point at - 1.5 MPa) field conditions. The dry moisture treatments were then combined with a rewet treatment where they were either rewet or maintained dry (+ or – rewet), resulting in a total of five dry/rewet treatments. Soils were packed into funnel tops to a BD of 1.1 g/cm³ and sealed in glass jars fitted with septa to allow gas sampling. Drying was achieved using silica gel which allowed continued gas measurement during drying periods. Gas samples were collected throughout the experiment and analysed for CO₂ by IRGA and N₂O by GC. At the start and end of the study, soils were analysed for Min N, MBC, MBN, HWC, DOC, POM, total C and total N. The correlation between calculated and actual C mineralisation data indicates that the intercept is not consistent with the origin and that the slope is not consistent with the 1:1 line. While those paddocks with high %C had high cumulative C mineralisation, there didn’t appear to be any strong relationship between soil texture or OM content and the difference between actual and calculated C mineralisation. A plot of calculated C mineralisation rates against the actual C mineralisation rates shows that much of the error in the calculated cumulative data arises from an underestimation of the mineralisation flush when the dry soil is rewetted, especially during the first dry-rewet cycle, and an over estimation of the rate at which respiration decreases as the soil dries. In order to use C mineralisation data from soils held at constant moisture contents to accurately predict C mineralisation in soils exposed to dry-rewet cycles, knowledge of the stress history for the soil would be required e.g. size, duration and frequency of rainfall events, dry rates etc. The N₂O-N emission data is inherently more variable than the C mineralisation data. The fine-textured soils tend to have much higher N₂O-N emissions than the coarser soils, probably due to the creation of anoxic sites upon rewetting in the fine-textured soils. The data indicates that prediction of N₂O-N emissions in soils exposed to dry-rewet cycles using emission data from soils held at constant moisture contents would be very inaccurate, primarily due to the inherent variability of N₂O-N emissions in soils.