Determination of litter derived C and N in litterbags and soil using stable isotopes prevents overestimation of litter decomposition in alley cropping systems
Introduction
Plant litter decomposition and related processes, like nutrient mineralization and carbon sequestration, are important ecosystem services (Bradford et al., 2016). Litter decomposition is regulated by the activity of soil organisms and abiotic factors, including climate, water and nutrient status of soils as well as resource quality (Aerts, 1997; Adair et al., 2008; Wall et al., 2008; Tresch et al., 2019). Regulating factors widely interact and their effects vary depending on the site and scale (García-Palacios et al., 2013). Thus, litter decomposition is a valuable integrative parameter for estimating overall biological activity. The litterbag method is based on the determination of mass loss from litter confined in mesh bags. It represents a widely used procedure for estimating decomposition rates under field conditions. Mass loss rates of litter are due to catabolism, leaching and fragmentation (Cotrufo et al., 2010). However, most estimates of litter decomposition rates do not account for litter mass loss rates due to litter transferred to the soil below litterbags. The access of soil macrofauna increased the translocation of organic matter into the mineral layer but did not significantly increase carbon mineralization (Frouz et al., 2006), demonstrating that mass loss rates may overestimate litter decomposition. Soong et al. (2016) estimated the transfer of C derived litter, like fragmented litter, soluble litter compounds, microbial biomass and litter derived microbial residues, to the soil below surface placed litter. They showed that more labile litter inputs are incorporated in the soil at early-stage decomposition when microarthropods are present. Litter may fall through the mesh, due to increasing comminution, or be directly transported out of the litterbags. The latter effect is caused by consumption of litter in the bags and faunal defecation of incompletely digested litter outside the litterbags. This is probably a relevant process, as C use efficiency of fauna is moderate (David, 2014) or even low (Lavelle et al., 1997; Coulis et al., 2013). Beside fragmentation loss, leaching of dissolved organic matter (DOM) may account for C and N losses from litterbags (Wachendorf et al., 1997). Subsequently, DOM is bound to mineral surfaces of the soil and stabilized, as evidenced by a decreased carbon mineralization of DOM in forest soils (Kalbitz et al., 2005). All of the above mentioned processes, leading to higher transfer rates of litter derived C to the mineral soil, have the potential to increase C sequestration. Thus, the analysis of remaining litter in soils may be a suitable indicator for C sequestration.
Tracking litter derived C may be achieved by application of 13C labeled litter to an unlabeled soil (Rubino et al., 2010; Helgason et al., 2014; Soong et al., 2016). Alternatively, C3 and C4 plants differ naturally in 13C signature due to difference in discrimination of 13C during CO2 assimilation (Balesdent and Mariotti, 1996). Therefore, litter derived C can be traced by adding plant litter from maize, a C4 plant, to a soil, with SOM primarily derived from C3 plants (Struecker et al., 2016). Furthermore, by application of 13C and 15N labeled litter different transfer rates to the soil of both elements can be traced simultaneously (Soong et al., 2016). To account for litter transferred from litterbags to the soil, we applied maize litter with a 13C and 15N signature, differing from that of the soil. In our experiment, the analysis of 13C in litter remaining in bags and in soil below litterbags enabled us to estimate decomposition of litter by correcting mass loss rates in litterbags for litter C and N transferred to the soil.
In alley cropping systems (ACS), annual agronomic crops or grassland are cultivated between rows of trees (Garrett and Buck, 1997). ACS are considered as a sustainable land-use system, according to decreased management intensity, as fertilizers are not usually applied to trees. Furthermore, deep rooting trees may increase nutrient cycling of alleyway soils. Trees in agroforestry systems indirectly modify the soil organism community by affecting habitat structure and soil chemical and physical properties (Jose, 2009; Quinkenstein et al., 2009). Therefore, changes in activity and composition of the soil organism community as well as carbon and nutrient availability in ACS have been observed (Cardinael et al., 2017; Beuschel et al., 2019; Beuschel et al., 2020a; Beule et al., 2019). However, processes like litter decomposition have been investigated less intensively in temperate ACS. In these systems, litter decomposition may be affected by inherent spatial heterogeneity as well as by modifications of soil habitat due to the implementation of trees. The effect of trees may vary, depending on distance, height and age of the plants. Thus, for the evaluation of tree effects on biological activity, the application of an appropriate statistical model, allowing the consideration of spatial dependence within ACS, is indispensable (Piepho and Edmondson, 2018). We therefore analyzed our data according to the approach reported by Beuschel et al. (2019). With this approach spatial dependence (e.g. serial correlation of observations within a transect) as well as different variance-covariance structures, clay contents and pH values, were considered within and between distance transects.
The aim of our study was: 1. to estimate litter decomposition of applied litter in coarse mesh sized litterbags by accounting for C and N transferred to the soil below litterbags, and 2. to quantify the effect of trees in ACS on these processes by considering tree distance and soil variability.
Section snippets
Experimental sites and soils
The study was conducted at five German ACS (Table 1), with woody biomass harvested in short-term cycles for bioenergy production. At three ACS (Forst, Dornburg and Wendhausen), poplars (clone Max 1) were planted in 12-m-wide alleys (Fig. 1A). In between, 48-m-wide alleyways with cereal-based crop rotations were run. Reduced tillage using chisel plough and rotary harrow was conducted. At Reiffenhausen, an arable ACS (Reiffenhausen-A) and a grassland ACS (Reiffenhausen-G) were established in
Results
Recovery of total litter derived C was lower under trees than in arable alleyways at all arable ACS (Fig. 2), indicating higher decomposition rates under trees. However, higher decomposition rates under trees were associated with a higher recovery of litter derived C in soil below the litterbags. At the grassland ACS, litter C remaining in bags and litter recovered in the soil did not differ between tree rows and alleyways, indicating similar effects of trees and permanent grassland on litter
Land-use and management effects on litter decomposition
Higher decomposition rates of litter under trees than under arable crops may have resulted from a longer period of root growth of trees and, thus, higher rhizodeposition inducing priming effects (Pausch et al., 2013). Priming effects in the presence of living roots may not only increase the decomposition rate of SOM (Kuzyakov, 2002), but also increase decomposition of plant residues (Wang et al., 2015). Soil tillage in the alleyways of arable ACS may have negatively affected litter
Conclusions
Transfer of litter C and N by fragmentation losses or leaching are often not accounted for in litterbag experiments. Our results from four arable and one grassland ACS showed that degradation of litter is overestimated by between 10 and 15%, when transferred litter C below litterbags is not considered. Nevertheless, the loss by fragmentation, caused by incomplete assimilation of litter derived C, is probably higher, considering that we only used 2 mm mesh sizes. Higher decomposition rates of
Declaration of Competing Interest
All authors have seen and approved the final version of the manuscript being submitted. The article is the authors' original work and hasn't received prior publication and isn't under consideration for publication elsewhere. The authors declare that they have no competing interests. The Journal policies detailed in the guide of Pedobiologia have been reviewed and considered.
Acknowledgments
Research was funded by the Federal Ministry of Education and Research (BMBF), Germany in the framework of BonaRes within the project “SIGNAL – sustainable intensification of agriculture through agroforestry”. We thank Mick Locke for English language editing.
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