The impact of altered management on long-term agricultural soil carbon stocks

Land use in general and particularly agricultural practices can significantly influence soil carbon storage. In this paper, we investigate the long-term effects of management changes on soil carbon stock dynamics on a Swedish farm where C concentrations were measured in 1956 at 124 points in a regular grid. The soil was re-sampled at 65 points in 1984 and at all grid points in 2001. Before 1956 most of the fodder for dairy cattle was produced on the farm and crop rotations were dominated by perennial grass leys and spring cereals with manure addition. In 1956 all animals were sold, crop rotations were thereafter dominated by wheat, barley and rapeseed. Spatial variation in topsoil C concentration decreased significantly between 1956 and 2001. C stocks declined in fields with initially large C stocks but did not change significantly in fields with moderate C stocks. In the latter fields, soil C concentrations declined from 1956 to 1984, but increased slightly thereafter according to both measurements and simulations. Thus, the decline in C input due to the altered management in 1956 was partly compensated for by increasing crop yields and management changes, resulting in increased C input during the last 20 years. A soil carbon balance model (ICBM) was used to describe carbon dynamics during 45 years. Yield records were transformed to soil carbon input using allometric functions. Topsoil C concentrations ranging between 1.8 and 2.4% (depending on individual field properties) seemed to be in dynamic equilibrium with C input under recent farming and climatic conditions. Subsoil C concentrations seemed to be unaffected by the management changes

Soil microbial biomass and community structure affected by repeated additions of sewage sludge in four Swedish long-term field experiments

Soil organic matter is a key attribute of soil fertility. The pool of soil organic C can be increased, either by mineral fertilisers or by adding organic amendments such as sewage sludge. Sewage sludge has positive effects on agricultural soils through the supply of organic matter and essential plant nutrients, but sludge may also contain unwanted heavy metals, xenobiotic substances and pathogens. One obvious effect of long-term sewage sludge addition is a decrease in soil pH, caused by N mineralisation followed by nitrification, sulphate formation and presence of organic acids with the organic matter added. The objective of this study was to investigate the effect of sewage sludge on the microbial biomass and community structure

Four Swedish long-term field experiments with sewage sludge reveal a limited effect on soil microbes and on metal uptake by crops

Purpose: This study aims to study the effect of sewage sludge amendment on crop yield and on microbial biomass and community structure in Swedish agricultural soils. Materials and methods Topsoil samples (0-0.20 m depth) from four sites where sewage sludge had been repeatedly applied during 14-53 years were analysed for total C, total N, pH and phospholipid fatty acids (PLFAs). Heavy metals were analysed in both soil and plant samples, and crop yields were recorded. Results and discussion At all four sites, sewage sludge application increased crop yield and soil organic carbon. Sludge addition also resulted in elevated concentrations of some heavy metals (mainly Cu and Zn) in soils, but high concentrations of metals (Ni and Zn) in plant materials were almost exclusively found in the oldest experiment, started in 1956. PLFA analysis showed that themicrobial community structure was strongly affected by changes in soil pH. At those sites where sewage sludge had caused low pH, Gram-positive bacteria were more abundant. However, differences in community structure were larger between sites than between the treatments. Conclusions: At all four sites, long-term sewage sludge application increased the soil organic carbon and nitrogen content, microbial biomass and crop yield. Long-term sewage sludge application led to a decrease in soil pH. Concentrations of some metals had increased significantly with sewage sludge application at all sites, but the amounts of metals added to soil with sewage sludge were found not to be toxic for microbes at any site

Phosphorus fertilisation under nitrogen limitation can deplete soil carbon stocks: evidence from Swedish meta-replicated long-term field experiments

Increasing soil organic carbon (SOC) in agricultural soils can mitigate atmospheric CO2 concentration and also contribute to increased soil fertility and ecosystem resilience. The role of major nutrients in SOC dynamics is complex, due to simultaneous effects on net primary productivity (NPP) that influence crop residue carbon inputs and in the rate of heterotrophic respiration (carbon outputs). This study investigated the effect on SOC stocks of three different levels of phosphorus and potassium (PK) fertilisation rates in the absence of nitrogen fertilisation and of three different levels of nitrogen fertiliser in the absence of PK fertiliser. This was done by analysing data from 10 meta-replicated Swedish long-term field experiments (> 45 years). With N fertilisation, SOC stocks followed yield increases. However, for all PK levels, we found average SOC losses ranging from 0.04 +/- 0.09 Mg ha(-1) yr(-1) (ns) for the lowest to 0.09 +/- 0.07 Mg ha(-1) yr(-1) (p = 0.008) for the highest application rate, while crop yields as a proxy for carbon input increased significantly with PK fertilisation by 1, 10 and 15 %. We conclude that SOC dynamics are mainly output-driven in the PK-fertilised regime but mostly input-driven in the N-fertilised regime, due to the much more pronounced response of NPP to N than to PK fertilisation. It has been established that P rather than K is the element affecting ecosystem carbon fluxes, where P fertilisation has been shown to (i) stimulate heterotrophic respiration, (ii) reduce the abundance of arbuscular mycorrhizal fungi and (iii) decrease the crop root : shoot ratio, leading to higher root-derived carbon input. The higher export of N in the PK-fertilised plots in this study could (iv) have led to increased N mining and thus mineralisation of organic matter. More integrated experiments are needed to gain a better understanding of the relative importance of each of the above-mentioned mechanisms leading to SOC losses after P addition

Qualitative and quantitative response of soil organic carbon to 40 years of crop residue incorporation under contrasting nitrogen fertilisation regimes

Crop residue incorporation (RI) is recommended to increase soil organic carbon (SOC) stocks. However, the positive effect on SOC is often reported to be relatively low and alternative use of crop residues, e.g. as a bioenergy source, may be more climate smart. In this context, it is important to understand: (i) the response of SOC stocks to long-term crop residue incorporation; and (ii) the qualitative SOC change, in order to judge the sustainability of this measure. We investigated the effect of 40 years of RI combined with five different nitrogen (N) fertilisation levels on SOC stocks and five SOC fractions differing in turnover times on a clay loam soil in Padua, Italy. The average increase in SOC stock in the 0–30cm soil layer was 3.1Mgha–1 or 6.8%, with no difference between N fertilisation rates. Retention coefficients of residues did not exceed 4% and decreased significantly with increasing N rate (R2=0.49). The effect of RI was higher after 20 years (4.6Mgha–1) than after 40 years, indicating that a new equilibrium has been reached and no further gains in SOC can be expected. Most (92%) of the total SOC was stored in the silt and clay fraction and 93% of the accumulated carbon was also found in this fraction, showing the importance of fine mineral particles for SOC storage, stabilisation and sequestration in arable soils. No change was detected in more labile fractions, indicating complete turnover of the annual residue-derived C in these fractions under a warm humid climate and in a highly base-saturated soil. The applied fractionation was thus useful to elucidate drivers and mechanisms of SOC formation and stabilisation. We conclude that residue incorporation is not a significant management practice affecting soil C storage in warm temperate climatic regions

Positive trends in organic carbon storage in Swedish agricultural soils due to unexpected socio-economic drivers

Soil organic carbon (SOC) plays a crucial role in the global carbon cycle as a potential sink or source. Land management influences SOC storage, so the European Parliament decided in 2013 that changes in carbon stocks within a certain land use type, including arable land, must be reported by all member countries in their national inventory reports for greenhouse gas emissions. Here we show the temporal dynamics of SOC during the past 2 decades in Swedish agricultural soils, based on soil inventories conducted in 19881997 (Inventory I), 2001-2007 (Inventory II) and from 2010 onwards (Inventory III), and link SOC changes with trends in agricultural management. From Inventory I to Inventory II, SOC increased in 16 out of 21 Swedish counties, while from Inventory I to Inventory III it increased in 18 out of 21 counties. Mean topsoil (0-20 cm) SOC concentration for the entire country increased from 2.48 to 2.67% C (a relative increase of 7.7 %, or 0.38% yr(-1)) over the whole period. We attributed this to a substantial increase in ley as a proportion of total agricultural area in all counties. The horse population in Sweden has more than doubled since 1981 and was identified as the main driver for this management change (R-2 = 0.72). Due to subsidies introduced in the early 1990s, the area of long-term set-aside (mostly old leys) also contributed to the increase in area of ley. The carbon sink function of Swedish agricultural soils demonstrated in this study differs from trends found in neighbouring countries. This indicates that country-specific or local socio-economic drivers for land management must be accounted for in larger-scale predictions

Relations between soil organic carbon content and the pore size distribution for an arable topsoil with large variations in soil properties

Soil organic carbon (SOC) in arable topsoil is known to have beneficial effects on soil physical properties that are important for soil fertility. The effects of SOC content on soil aggregate stability have been well documented; however, few studies have investigated its relationship with the soil pore structure, which has a strong influence on water dynamics and biogeochemical cycling. In the present study, we examined the relationships between SOC and clay contents and pore size distributions (PSDs) across an arable field with large spatial variations in topsoil SOC and clay contents by combining X-ray tomography and measurements of soil water retention. Additionally, we investigated the relationships between fractionated SOC, reactive Fe and Al oxide contents and soil pore structure. We found that porosities in the 0.2-720 mu m diameter class were positively correlated with SOC content. A unit increase of SOC content was associated with a relatively large increase in porosity in the 0.2-5 and 480-720 mu m diameter classes, which indicates that enhanced SOC content would increase plant available water content and unsaturated hydraulic conductivity. On the other hand, macroporosities (1200-3120 mu m diameter classes) and bioporosity were positively correlated with clay content but not with SOC content. Due to strong correlations between soil texture, carbon-to-nitrogen ratios and reactive iron contents, we could not separate the relative importance of these soil properties for PSDs. Reactive aluminium and particulate organic carbon contents were poorer predictors for PSDs compared with clay and SOC contents. This study provides new insights on the relations between SOC and soil pore structure in an arable soil and may lead to improved estimations of the effects of enhanced SOC sequestration on soil water dynamics and soil water supply to crops. Highlights Relations between soil organic carbon (SOC) and pore size distribution (PSD) in an arable soil were explored. We used X-ray tomography and soil water retention to quantify a wide range of PSD. There were positive correlations between SOC and porosities in 0.2-720 mu m diameter classes. Porosities in 0.2-5 and 480-720 mu m diameter classes were more strongly correlated with SOC than clay. Our results have implications for improved estimates of effects of SOC sequestration on soil water dynamics

Net primary productivity and below-ground crop residue inputs for root crops: Potato (Solanum tuberosum L.) and sugar beet (Beta vulgaris L.)

Root crops are significant in agro-ecosystems of temperate climates. However, the amounts of crop residues for these crop types are not well documented and they need to be accounted for in the modeling of soil organic carbon dynamics. Our objective was to review field measurements of root biomass left in the soil as crop residues at harvest for potato and sugar beet. We considered estimates for crop residue inputs as root biomass presented in the literature and some unpublished results. Our analysis showed that compared to, for example, cereals, the contribution of below-ground net primary productivity (NPP) to crop residues is at least two to three times lower for root crops. Indeed, the field measurements indicated that root biomass for topsoils only represents on average 25 to 30 g dry matter (DM) m(-2) yr(-1). Other estimates, albeit variable and region-specific, tended to be higher. We suggest relative plant DM allocation coefficients for agronomic yield (R-P), above-ground biomass (R-S) and root biomass (R-R) components, expressed as a proportion of total NPP. These coefficients, representative for temperate climates (0.739:0.236:0.025 for potato and 0.626:0.357:0.017 for sugar beet), should be useful in the modeling of agro-ecosystems that include root crops