Understanding, quantifying and managing soil carbon dynamics are areas of research vital in the context of terrestrial feedbacks to climate change, and to the sustainability of soils for provision of diverse ecosystem services. Maintenance of soil organic matter (SOM) is essential for soil structure to support storage of water, aeration and resistance to erosion, while SOM also represents the dominant stock of elements required for plant growth. Both the retention and loss of SOM is dependent on biotic processes, being inherent to the metabolic processes that transform and consume plant inputs. These fluxes of plant-derived C through soil food webs are also intimately coupled to soil nutrient cycling, and therefore, constitute feedbacks that mediate plant competition and ecosystem productivity (Fischer et al. 2014). These plant-soil interactions are complex, are affected by environmental variables, and outcomes for soil carbon (C) stocks are difficult to predict, or indeed to quantify (Kopittke et al. 2013; Haugwitz et al. 2014; Peuhl et al. 2012). Research directed to resolving these uncertainties is central to the remit of Plant and Soil and encompasses biophysical and modelling approaches.
Soil C balance is determined by the relative magnitudes of inputs from plants; in the forms of litter, crop residues, root turnover and rhizodeposition; and of losses resulting from SOM mineralisation, leaching of dissolved organic matter and soil erosion. Quantification of plant inputs, particularly belowground, remains a research challenge, but is increasingly being understood through use of isotopic labelling approaches (Pausch et al. 2013; Fahey et al. 2013; Tian et al. 2013a). The quantity and quality of these inputs (e.g. recalcitrance and C-to-nitrogen (N) ratio) differ between ecosystems and subsequently strongly affect microbial communities and nutrient cycling processes (Sanaullah et al. 2014; Ampleman et al. 2014). At a finer level, C-deposition varies as functions of crop type and genotype (Fischer et al. 2014), leading to increasing interest in the potential of crop selection and rotation management as a means to promote soil C-storage (Redin et al. 2014). This can be seen as exploiting the capacity of soils to sequester C, particularly those where stocks have been depleted due to past intensive management (Lockwell et al. 2012). For agricultural systems, the balance between C-storage and loss is complex, particularly when considered in the context of GHG mitigation and crop productivity. C-accrual is seen as having multiple benefits, including net removal of CO2 from the atmosphere; however, it is microbial turnover of SOM (i.e. consumption) that mobilises nutrients supporting plant growth and potentially reduces fertiliser requirements. Therefore, research areas are developing that consider integrated approaches to management of SOM, through rotations (Kong and Six 2012; Fuentes et al. 2012; Tian et al. 2013b), management (Steffens et al. 2011), use of organic fertilisers (Benbi et al. 2012), tillage practice (Sun et al. 2011; Fuentes et al. 2012; Álvaro-Fuentes et al. 2014) and soil amendments (e.g. biochar, Atkinson et al. 2010; Scheer et al. 2011; Zhao et al. 2014).
Interactions between chemical, physical and biological processes are critical to the understanding of soil C-dynamics (Fonte et al. 2012; Pérès et al. 2013), as are how these interactions shift as a consequence of climate change (Haugwitz et al. 2014; Sanaullah et al. 2014). Currently there are series’ of parallel reports demonstrating the importance of chemical recalcitrance and soil aggregation (Qiu et al. 2012; Pérès et al. 2013) and interactions with mineral surfaces (Schöning et al. 2013) as limitations to SOM decomposition; a current challenge is to consider each of these factors in common systems to establish their relative importance. It is notable that SOM decomposition in subsoils may be regulated by different mechanisms than in more active root-zones (Sanaullah et al. 2011), meaning that the contribution of subsoils to soil-atmosphere C-exchanges may require specific attention (Harper and Tibbett 2013), but also highlighting an opportunity to further probe the controls of SOM decomposition (Rumpel and Kögel-Knabner 2011). Further, recent studies have highlighted that rates of SOM decomposition are affected by the availability of labile (plant-derived) C to microbial communities (i.e. priming effects, Carrillo et al. 2011; Sanaullah et al. 2014), suggesting that simple relationships between plant C-deposition rates and soil C-accrual may not be valid. Therefore, continued development of soil C models is required to incorporate emerging mechanistic understanding of processes, their controls and their interactions (Blagodatsky et al. 2011). This is particularly important for prediction of soil-atmosphere feedbacks to climate change, a recognised limitation of current general circulation models (GCMs), and should include potential interactions with fluxes of other radiatively active gases (N2O and CH4, Shvaleva et al. 2014). Effective progress in this important and fascinating research area will require work at a range of spatial scales and contributions across biophysical disciplines to resolve key plant-soil interactions mediating soil C-cycling processes.
Eric Paterson, Section Editor