Elsevier

Geoderma

Volume 303, 1 October 2017, Pages 93-98
Geoderma

Modelling soil organic matter dynamics on a bare fallow Chernozem soil in Central Germany

https://doi.org/10.1016/j.geoderma.2017.05.013Get rights and content

Highlights

  • Modelling of SOC turnover on bare fallow including soil physical effects

  • Comparing three decades of bare fallow with and without soil tillage

  • Model predictions for SOC and TN

Abstract

Soil organic matter (SOM) can be characterised by soil organic carbon (SOC) and/or total nitrogen (TN). The observed dynamics of SOC and TN in the topsoil of a 28-year-old fallow experiment on Haplic Chernozem was modelled using the Candy Carbon Balance (CCB) model. This study selected two treatments from this experiment where the soil was kept bare with mechanical or chemical (herbicides) treatments. The CCB model was improved to include the SOC related change of soil physical parameters and dynamic handling of the physically stabilised SOM pool. Over 28 years of bare fallow the top soil lost about 10 t/ha of SOC and > 1 t/ha of TN. The results from observation and modelling reflected the increased SOM turnover due to soil tillage. The modelled size of the physically stabilised SOC pool was about 55% of total SOC and only reduced slowly during the almost three decades, but the implementation of this effect improved simulation results and reduced the relative RMSD (unitless) from 0.051 to 0.044 for SOC and from 0.053 to 0.049 for TN error level. From these results we conclude that the larger the SOM change the more important is the integration of the turnover of physically stabilised SOM within the modelling approach.

Introduction

Soil and soil functions are gaining increasing attention because healthy soil is a fundamental requirement for sustainable development. As the largest terrestrial biotic carbon pool (Stockmann et al., 2013), SOM is a particular focus of the global change debate. SOM is a driver for important soil functions like carbon storage and nutrient release. However, SOM is affected by global change due to the interactions with climate conditions and changes of land management. Therefore, modelling is widely used to predict possible impacts of land use changes on SOM storage in search for carbon sequestration strategies or adaptation measures, especially regarding agroecosystems.

Most SOM models distribute the organic matter (OM) of the soil between several conceptual pools with specific turnover times to reflect the observed SOM dynamics on long-term experiments where, in most cases only SOC is used to indicate the quantitative changes while the N component of SOM is not considered. If the turnover time of a pool is very high or tending to infinity, it may be considered as inert or more generally, as being stabilised long-term. In common agricultural systems this long-term stabilised SOC represents the basic level above which the SOC observations fluctuate, representing the dynamics of the more labile pools. On a bare fallow treatment these more labile SOM pools are continuously depleted, and the observable SOC dynamics are increasingly dominated by the properties of the stabilised SOM pools. Hence, SOC dynamics on bare fallow treatments are considered suitable to analyse the behaviour of the long-term stabilised SOM pool (Barré et al., 2010, Menichetti et al., 2015).

A special fallow experiment on a Chernozem soil was started in 1988 in Bad Lauchstädt, Germany including treatments to study the effects of keeping the soil bare by either tillage or herbicide application. The SOM data from this experiment were used to model the behaviour of stabilised SOM on these bare fallow treatments and to review the assumptions about the tillage effect on SOM turnover already implemented in the CCB model (Franko and Spiegel, 2016).

In general, the CCB model (Franko et al., 2011) considers three pools of SOM: active SOM (ASOM), stabilised SOM (SSOM), and long-term stabilised SOM (LTS). These SOM pools can be combined with different pools of fresh organic matter according to the land use. Site-specific turnover is simulated using the concept of Biologic Active Time (Franko and Oelschlägel, 1995), which is similar to the use of the site-specific rate modifier within ICBM (Introductory Carbon Balance Model) of Andrén and Kätterer (1997).

So far, the CCB model concept has considered the LTS pool as not taking part in the turnover processes. Following the concept of Kuka et al. (2007), the calculation of the LTS pool size is based on indicators of soil structure given by the hydrological soil characteristics such as pore volume, field capacity, and permanent wilting point (Θpwp) as reported by Puhlmann et al. (2006). These hydrological soil characteristics are influenced by soil texture, SOC concentration, and bulk density (BD). We therefore hypothesise that BD and Θpwp, which depend on soil texture and SOC, are the main drivers for changes in the LTS pool size. In many cases, these physical soil properties are handled as parameters that don't change over the investigated time. This might be reasonable when looking at short time scales and moderate SOC changes that are typical for many agroecosystems. However, this assumption is not reasonable in case of an extraordinary SOC variation after land use changes from normal agriculture to bare fallow. Despite the known dependence of BD and Θpwp from SOC (e.g. Körschens et al., 1995), it remains an open question to what degree a change of soil physical properties influences the observable SOM dynamics in terms of SOC and TN. Therefore, we included both elements (C and N) in the assessment of the model results and implemented an additional sub model in CCB that adapts BD and Θpwp to the current SOC level and changes the LTS pool size according to the actual soil physical parameters.

The objective of this study was to assess the performance of the extended CCB model to predict the dynamics of SOC and TN in general, to analyse the dimension of the modelled LTS pool change and the tillage impact on SOM dynamics under bare fallow. Furthermore, we compared our results with results from Barré et al. (2010) where the SOC under bare fallow at several sites was described by an exponential function.

Section snippets

Experimental design

In this study data from a field experiment situated on a Haplic Chernozem soil in Bad Lauchstädt, Central Germany (51°24′N, 11°53′E) was used. The climate is semi-humid with a mean annual air temperature of 8.9 °C and 481 mm mean annual precipitation for the last three decades.

The experiment was started in 1988 to study different fallow treatments: mechanical bare fallow (MBF) keeping the soil bare by tillage, chemical bare fallow (CBF) keeping the soil bare by herbicide application, the

Performance of the CCB model

The results showed that the extended CCB model was able to reproduce the observed dynamics for both elements SOC and TN (Fig. 1). In average of both treatments, the relative error RMSDrel was 0.044 and 0.049 for SOC and TN, respectively. Without consideration of LTS dynamics (not shown), the relative error was 0.051 and 0.053 for SOC and TN, respectively. Furthermore, the now improved model reproduced the observations with an error near to the relative observation error (coefficient of

Model representation of bare fallow

The introduction of a bare fallow is a very intensive change of land use. Microbiological investigation showed significant differences between bare fallow and cultivated treatments (Nunan et al., 2015). The CCB model did not require adaptations of the turnover parameters of easily decomposable soil organic matter. Therefore, we can assume that the representation of turnover the processes in the CCB model with its conceptual pools is not depending on the microbial composition in the soil.

From a

Conclusions

The fitting results underline that the model was successfully applied to bare fallow treatments on Haplic Chernozem to represent the dynamics of SOC and TN in topsoil, including the effect of different soil management (MBF vs. CBF). The results confirm that the model concept to represent tillage effects (Franko and Spiegel, 2016) that was validated at the Fuchsenbigl tillage experiment in Austria is also applicable to different site conditions. The handling of physical soil properties as

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