Detecting small scale spatial heterogeneity and temporal dynamics of soil organic carbon (SOC) stocks: a comparison between automatic chamber derived C budgets and repeated soil inventories
Abstract. Carbon (C) sequestration in soils plays a key role in the global C cycle. It is therefore crucial to adequately monitor dynamics in soil organic carbon (DSOC) stocks when aiming to reveal underlying processes and potential drivers. However, small scale spatial (10 30 m) and temporal changes in SOC stocks, particularly pronounced in arable lands, are hard to assess. The main reasons for this are limitations of the well established methods. On the one hand, repeated soil inventories, often used in long term field trials, reveal spatial patterns and trends in DSOC but require a longer observation period and a sufficient number of repetitions. On the other hand, eddy covariance measurements of C fluxes towards a complete C budget of the soil plant atmosphere system may help to obtain temporal DSOC patterns but lack small scale spatial resolution. To overcome these limitations, this study presents a reliable method to detect both short term temporal dynamics as well as small scale spatial differences of DSOC using measurements of the net ecosystem carbon balance (NECB) as a proxy. To estimate the NECB, a combination of automatic chamber (AC) measurements of CO2 exchange and empirically modeled aboveground biomass development (NPPshoot) were used. To verify our method, results were compared with DSOC observed by soil resampling. Soil resampling and AC measurements were performed from 2010 to 2014 at a colluvial depression located in the hummocky ground moraine landscape of northeastern Germany. The measurement site is characterized by a variable groundwater level (GWL) and pronounced small scale spatial heterogeneity regarding SOC and nitrogen (Nt) stocks. Tendencies and magnitude of DSOC values derived by AC measurements and repeated soil inventories corresponded well. The period of maximum plant growth was identified as being most important for the development of spatial differences in annual DSOC. Hence, we were able to confirm that AC based C budgets are able to reveal small scale spatial differences and short term temporal dynamics of DSOC.