48 research outputs found

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

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    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

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

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    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

    Root litter quality drives the dynamic of native mineral-associated organic carbon in a temperate agricultural soil

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    Background and aims: Understanding the fate and residence time of organic matter added to soils, and its effect on native soil organic carbon (SOC) mineralisation is key for developing efficient SOC sequestration strategies. Here, the effect of litter quality, particularly the carbon-to-nitrogen (C:N) ratio, on the dynamics of particulate (POC) and mineral-associated organic carbon (MAOC) were studied. Methods: In a two-year incubation experiment, root litter samples of the C4-grass Miscanthus with four different C:N ratios ranging from 50 to 124 were added to a loamy agricultural topsoil. In an additional treatment, ammonium nitrate was added to the C:N 124 litter to match the C:N 50 litter input ratio. Soils were size-fractionated after 6, 12 and 24 months and δ13^{13}C was measured to determine the proportion of new and native POC and MAOC. Litter quality was further assessed by mid-infrared spectroscopy and compound peak analysis. Results: Litter quality strongly affected SOC dynamics, with total SOC losses of 42.5 ± 3.0% in the C:N 50 treatment and 48.9 ± 3.0% in the C:N 124 treatment after 24 months. Largest treatment effects occurred in mineralisation of native MAOC, which was strongly primed by litter addition. The N amendment in the C:N 124 treatment did not alleviate this potential N mining flux. Conclusion: Litter quality plays a major role in overall SOC dynamics, and priming for N mining from the MAOC pool could be a dominant mechanism. However, adding N did not compensate for poor litter quality, highlighting the role of litter quality beyond stoichiometric imbalances

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

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    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

    Deforestation‐free land‐use change and organic matter‐centered management improve the C footprint of oil palm expansion

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    In recent decades, mounting evidence has indicated that the expansion of oil palm (OP) plantations at the expense of tropical forest has had a far pernicious effect on ecosystem aspects. While various deforestation-free strategies have been proposed to enhance OP sustainability, field-based evidence still need to be consolidated, in particular with respect to savanna regions where OP expansion has recently occurred and that present large area with potential for OP cultivation. Here we show that the common management practice creating within the plantation the so-called management zones explained nearly five times more variability of soil biogeochemical properties than the savanna land-use change per se. We also found that clayey-soil savanna conversion into OP increased total ecosystem C stocks by 40 ± 13 Mg C ha(−1) during a full OP cultivation cycle, which was due to the higher OP-derived C accumulated in the biomass and in the soil as compared to the loss of savanna-derived C. In addition, application of organic residues in specific management zones enhanced the accumulation of soil organic carbon by up to 1.9 Mg ha(−1) year(−1) over the full cycle. Within plantation, zones subjected to organic amendments sustained similar soil microbial activity as in neighboring savannas. Our findings represent an empirical proof-of-concept that the conversion of non-forested land in parallel with organic matter-oriented management strategies can enhance OP agroecosystems C sink capacity while promoting microbe-mediated soil functioning. Nonetheless, savannas are unique and threatened ecosystems that support a vast biodiversity. Therefore, we suggest to give priority attention to conservation of natural savannas and direct more research toward the impacts of the conversion and subsequent management of degraded savannas.ISSN:1354-1013ISSN:1365-248

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

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    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

    Oxalate-extractable aluminum alongside carbon inputs may be a major determinant for organic carbon content in agricultural topsoils in humid continental climate

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    The relative importance of various soil mineral constituents (e.g. clay-sized particles, aluminum- and iron-bearing mineral reactive phases) in protecting soil organic carbon (SOC) from decomposition is not yet fully understood in arable soils formed from quaternary deposits in humid continental climates. In this study, we investigated the relationships between soil physico-chemical properties (i.e. contents of oxalate-extractable aluminum (Alox) and iron (Feox) and clay size particle < 2 mu m), grain yield (as a proxy for carbon input) and total SOC as well as SOC in different soil fractions for samples taken from the topsoil of an arable field at Bjertorp in south-west Sweden. We found a positive correlation between Alox and total SOC content, where Alox explained ca. 48% of the spatial variation in SOC. We also found that ca. 80% of SOC was stored in silt- and claysized (SC) fractions, where Al-bearing reactive mineral phases (estimated by Alox) may be important for organicmineral associations and clay aggregation. Our results were supported by data collated from the literature for arable topsoil in similar climates, which also showed positive correlations between SOC and Alox contents (R-2 = 23.1 - 74.5%). Multiple linear regression showed that including spatially-variable crop yields as a proxy for carbon inputs improved the prediction of SOC variation across the Bjertorp field. Other unquantified soil properties such as exchangeable calcium may account for the remaining unexplained variation in topsoil SOC. We conclude that Al-bearing reactive mineral phases are more important than clay content and Fe-bearing reactive mineral phases for SOC stabilization in arable topsoil in humid continental climates

    Microbial carbon use efficiency along an altitudinal gradient

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    Soil microbial carbon-use efficiency (CUE), described as the ratio of growth over total carbon (C) uptake, i.e. the sum of growth and respiration, is a key variable in all soil organic matter (SOM) models and critical to ecosystem C cycling. However, there is still a lack of consensus on microbial CUE when estimated using different methods. Furthermore, the significance of many fundamental drivers of CUE remains largely unknown and inconclusive, especially for tropical ecosystems. For these reasons, we determined CUE and microbial indicators of soil nutrient availability in seven tropical forest soils along an altitudinal gradient (circa 900-2200 m a.s.l) occurring at Taita Hills, Kenya. We used this gradient to study the soil nutrient (N and P) availability and its relation to microbial CUE estimates. For assessing the soil nutrient availability, we determined both the soil bulk stoichiometric nutrient ratios (soil C:N, C:P and N:P), as well as SOM degradation related enzyme activities. We estimated soil microbial CUE using two methods: substrate independent O-18-water tracing and C-13-glucose tracing method. Based on these two approaches, we estimated the microbial uptake efficiency of added glucose versus native SOM, with the latter defined by 18O-water tracing method. Based on the bulk soil C:N stoichiometry, the studied soils did not reveal N limitation. However, soil bulk P limitation increased slightly with elevation. Additionally, based on extracellular enzyme activities, the SOM nutrient availability decreased with elevation. The C-13-CUE did not change with altitude indicating that glucose was efficiently taken up and used by the microbes. On the other hand, 18O-CUE, which reflects the growth efficiency of microbes growing on native SOM, clearly declined with increasing altitude and was associated with SOM nutrient availability indicators. Based on our results, microbes at higher elevations invested more energy to scavenge for nutrients and energy from complex SOM whereas at lower elevations the soil nutrients may have been more readily available.Peer reviewe

    Microbial carbon use efficiency along an altitudinal gradient

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    Soil microbial carbon-use efficiency (CUE), described as the ratio of growth over total carbon (C) uptake, i.e. the sum of growth and respiration, is a key variable in all soil organic matter (SOM) models and critical to ecosystem C cycling. However, there is still a lack of consensus on microbial CUE when estimated using different methods. Furthermore, the significance of many fundamental drivers of CUE remains largely unknown and inconclusive, especially for tropical ecosystems. For these reasons, we determined CUE and microbial indicators of soil nutrient availability in seven tropical forest soils along an altitudinal gradient (circa 900–2200 m a.s.l) occurring at Taita Hills, Kenya. We used this gradient to study the soil nutrient (N and P) availability and its relation to microbial CUE estimates. For assessing the soil nutrient availability, we determined both the soil bulk stoichiometric nutrient ratios (soil C:N, C:P and N:P), as well as SOM degradation related enzyme activities. We estimated soil microbial CUE using two methods: substrate independent 18O-water tracing and 13C-glucose tracing method. Based on these two approaches, we estimated the microbial uptake efficiency of added glucose versus native SOM, with the latter defined by 18O-water tracing method. Based on the bulk soil C:N stoichiometry, the studied soils did not reveal N limitation. However, soil bulk P limitation increased slightly with elevation. Additionally, based on extracellular enzyme activities, the SOM nutrient availability decreased with elevation. The 13C-CUE did not change with altitude indicating that glucose was efficiently taken up and used by the microbes. On the other hand, 18O-CUE, which reflects the growth efficiency of microbes growing on native SOM, clearly declined with increasing altitude and was associated with SOM nutrient availability indicators. Based on our results, microbes at higher elevations invested more energy to scavenge for nutrients and energy from complex SOM whereas at lower elevations the soil nutrients may have been more readily available
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