Fate of straw- and root-derived carbon in a Swedish agricultural soil

Fate of straw- and root-derived carbon in a Swedish agricultural soil Abdul Ghafoor 0 1 Christopher Poeplau 0 1 Thomas Kätterer 0 1 0 Thuenen Institute of Climate-Smart Agriculture , Bundesallee 50, 38116 Braunschweig , Germany 1 Department of Ecology, Swedish University of Agricultural Sciences (SLU) , Box 7044, 75007 Uppsala , Sweden To maximise carbon (C) storage in soils, understanding the fate of C originating from aboveground and belowground residues and their interaction with fertiliser under field conditions is critically important. The use of 13C natural abundance provides unique opportunities to separate both C sources. We investigated the effect of 16 years of C3 straw and C4 root input, with and without nitrogen (N) addition, on SOC stocks and C distribution in soil fractions in the long-term frame trial at Ultuna, Sweden. The straw C input was fixed at 1.77 Mg ha−1 year−1, while the root input depended on maize plant growth, enabling studies on how N fertilisation affected (i) stabilisation of residues and (ii) plant C allocation to belowground organs. Four treatments were investigated: only maize roots (Control), maize roots with N (Control + N), maize roots and straw (Straw) and maize roots, straw and N (Straw + N). After 16 years, 5.6-8.9% of the total SOC stock in the 0-20 cm soil layer was maize-derived. In all four treatments, the relatively labile SOC fractions decreased, while the proportion of more refractory fractions increased. Based on allometric calculation of root inputs, retention of maize roots was 38, 26, 36 and 18% in the Control, Control + N, Straw and Straw + N treatments, respectively. The estimated retention coefficient of C3 straw in the Straw + N treatment was higher than that in the Straw-N treatment. We interpreted these results thus (1) roots were better stabilised in the soil than straw; (2) N fertilisation caused a shift in root to shoot ratio, with relatively more roots being present in Ndeficient soil; and (3) N fertilisation caused greater stabilisation of residues, presumably due to increased microbial C use efficiency. Carbon sequestration; Fractionation; Carbon modelling; Stable isotopes; C input - Soil organic matter (SOM) serves many ecosystem functions from being reservoir for nutrients to act as agent to increase biological activity, provides soil aggregation, retains moisture and improves soil structure and tilth for reducing soil erosion. Moreover, SOM comprises a significant part of the global terrestrial C pool. The world’s soils store at least three times as much C as is found in either the atmosphere or living plants (Lal 2004). A small change in the SOC pool has a critical influence on atmospheric CO2 concentration (Poeplau et al. 2011; von Lützow et al. 2006). Kell (2012) postulated that an overall increase in soil C of 10% would decrease atmospheric C by at least 20%. Thus, given the growing interest in increasing SOC stocks in soils world-wide to mitigate climate change and improve soil quality, better understanding of soil organic C (SOC) stabilisation and its dynamics in soil in response to various management practises is indispensable (Lal 2004). Conceptually, SOC is often partitioned into different pools/ fractions with distinct physico-chemical properties, different degrees of stabilisation and turnover times ranging from years to millennia (Bol et al. 2009; von Lützow et al. 2007). A variety of biochemical, physical and chemical processes protect organic C from decomposition in soils, and knowledge of the resulting persistence of SOC pools/fractions in soil is vital in understanding their contribution to the global C cycle. Stabilisation of SOC in soil depends on numerous factors such as soil type, climate, substrate quality, input pathway and nutrient regime (Kätterer et al. 2011; Kirkby et al. 2013). In a mechanistic perspective, four major mechanisms may explain the stability of SOM in soil: (i) spatial inaccessibility of SOM to decomposers due to aggregation, (ii) recalcitrance due to the chemical structure, (iii) stabilisation of SOM by interaction with mineral surfaces and (iv) energetical limitation microbes to decompose organic matter (Mueller et al. 2014; von Lützow et al. 2006; Fontaine et al. 2007). The distribution of organic matter between soil size fractions is affected differently when soil organic matter levels change due to cultivation, straw incorporation, addition of mineral fertiliser or animal manure (Christensen and Sorensen 1985; Christensen 1987; Gregorich et al. 1995). Using a detailed balancing approach, Kätterer et al. (2011) showed that root-derived C was preferentially retained in soil and contributed 2.3-fold more to the C pool than aboveground residues. Potential reasons for this, as elegantly summarised by Rasse et al. (2005), are as follows: (i) roots are relatively more recalcitrant than shoots; (ii) the physico-chemical protection of root C by aggregates and mineral surfaces is higher than for shoot (...truncated)