Deep-C storage: Biological, chemical and physical strategies to enhance carbon stocks in agricultural subsoils

Due to their substantial volume, subsoils contain more of the total soil carbon (C) pool than topsoils. Much of this C is thousands of years old, suggesting that subsoils offer considerable potential for long-term C sequestration. However, knowledge of subsoil C behaviour and manageability remains incomplete, and subsoil C storage potential has yet to be realised at a large scale, particularly in agricultural systems. A range of biological (e.g. deep-rooting), chemical (e.g. biochar burial) and physical (e.g. deep ploughing) C sequestration strategies have been proposed, but are yet to be assessed. In this review, we identify the main factors that regulate subsoil C cycling and critically evaluate the evidence and mechanistic basis of subsoil strategies designed to promote greater C storage, with particular emphasis on agroecosystems. We assess the barriers and opportunities for the implementation of strategies to enhance subsoil C sequestration and identify 5 key current gaps in scientific understanding. We conclude that subsoils, while highly heterogeneous, are in many cases more suited to long-term C sequestration than topsoils. The proposed strategies may also bring other tangible benefits to cropping systems (e.g. enhanced water holding capacity and nutrient use efficiency). Furthermore, while the subsoil C sequestration strategies we reviewed have large potential, more long-term studies are needed across a diverse range of soils and climates, in conjunction with chronosequence and space-for-time substitutions. Also, it is vital that subsoils are more consistently included in modelled estimations of soil C stocks and C sequestration potential, and that subsoil-explicit C models are developed to specifically reflect subsoil processes. Finally, further mapping of subsoil C is needed in specific regions (e.g. in the Middle East, Eastern Europe, South and Central America, South Asia and Africa). Conducting both immediate and long-term subsoil C studies will fill the knowledge gaps to devise appropriate soil C sequestration strategies and policies to help in the global fight against climate change and decline in soil quality. In conclusion, our evidence-based analysis reveals that subsoils offer an untapped potential to enhance global C storage in terrestrial ecosystems

Addition of iron to agricultural topsoil and subsoil is not an effective C sequestration strategy

The interaction of soil organic matter (SOM) with Fe-containing minerals represents a key mechanism that promotes carbon (C) stabilisation in soil. The addition of Fe-rich industrial by-products to soil may therefore help accelerate C storage. Our understanding of the effects of exogenous Fe addition (Fe (oxy)hydroxide vs. Fe chloride) on SOM dynamics and C dynamics in agricultural soils, especially in subsoils, however, remains poor. Here, we simulate the addition of Fe in an arable soil context and assess its effectiveness based on CO2 emissions and soil chemistry. We hypothesised that insoluble and soluble Fe would reduce the mineralization of newly added unprotected organic materials more than native SOM and that soluble Fe would cause mineralisation of native SOM. To investigate this, insoluble Fe(OH)3 or soluble FeCl2 (0–5 g kg−1) were added to arable top- (0–10 cm) or subsoils (50–60 cm) and CO2 emissions, pH and nutrient dynamics (e.g. P, N) measured in a laboratory incubation over a 45 d period. We also compared the effect of Fe on the turnover of native SOM and newly added C (i.e. 14C-labelled glucose, citrate and crop residues) which was pre-mixed with exogenous Fe. We found that: (1) despite a reduction in P and DOC, Fe(OH)3 did not suppress total CO2 efflux; (2) high FeCl2 rates induced a rapid and significant release of CO2, which we attribute almost entirely to FeCl2-induced soil acidification increasing DOC availability and carbonate dissolution; (3) 14C-substrate mineralisation was weakly suppressed by Fe(OH)3 but strongly by FeCl2 following the series: citrate < glucose < crop residues; and (4) Fe addition to subsoils induced a stronger C mineralisation response but weaker effect on soil solution chemistry compared to topsoil, possibly due to subsoils having a lower buffering ability and less microbial biomass. We conclude that addition of extra Fe was not effective in promoting greater C sequestration in the arable soil we tested

Recomposition: Coordinating a Web of Software Dependencies

Abstract. In this paper, I revisit the concept of recomposition – all the work that development organizations do to make sure that their product fits together and into a broader environment of other technologies. Technologies, such as Configuration Management (CM) systems, can ameliorate some of a software development team’s need to engage in recomposition. However, technological solutions do not scale to address other kinds of recomposition needs. This paper focuses on various organizational responses to the need for recomposition. By organizational response, I mean how individuals engage in recomposition so that the organization can assemble software systems from parts. Specifically, I describe how those responses are manifested in the day-to-day communications and responsibilities of individuals throughout the organization. I also highlight how changes in an organization complicate recomposition. The paper concludes with a discussion of three features of software development work that are revealed by recomposition: the affects of environmental disturb-ances on development work, the types of dependencies that require recomposition, and the images of organizations required to manage the recomposition. Key words: empirical studies, recomposition, software development 1