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
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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)