Soil Organic C Mineralization Research Articles

Effects of aggregation processes on distribution of aggregate size fractions and organic C content of a long-term fertilized soil

Mineral and organic fertilizers are the important factors for maintenance and improvement of soil fertility and aggregation. Despite aggregation and aggregate stability are proxy of soil fertility, the connection between fertilization and aggregation is not direct, as short and long-term processes may affect the aggregate formation in different directions. In this study, the long-term effects of a 20-year application of mineral and organic fertilizers were studied in an intensive horticultural crop rotation with the following treatments: i) without fertilization (control soil), ii) nitrogen applied by mineral fertilizer, and iii) farmyard manure application with low (30 t ha −1 y −1) or iv) high (60 t ha −1 y −1) rates. In case of short-term aggregation process, K-polyacrylate was added to the soil to change aggregate composition and then the aggregated soils were incubated for 2 weeks. Long-term fertilization increased the soil organic C (SOC) content by 42–73% and the portion of small macroaggregates (1–0.25 mm) compared to control soil. In contrast, soil aggregation induced by K-polyacrylate showed an increase of the large macroaggregate (2–1 mm) portion independent of fertilization. Polyacrylate had no effect on soil microbial biomass C. According to the increased SOC content, the fertilization increased CO 2 efflux from soil (4.2–5.2% of SOC after 80 days of incubation). Short-term aggregation by K-polyacrylate decreased the SOC mineralization rate mainly of the labile C-pools. In conclusion the data of this study suggest that long-term fertilization mainly contributes to the formation of small macroaggregates. In contrast, the formation of large macroaggregates is mediated mainly by short-term processes and contributes to the decrease of SOC mineralization.

Effects of long-term chemical fertilization and organic amendments on dynamics of soil organic C and total N in paddy soil derived from barren land in subtropical China

For better understanding the development of infertile paddy soils in subtropical China, a long-term field experiment of paddy soil was developed from barren land in 1990. Experimental treatments including NPK, NPKRS (NPK and rice straw), NPK2RS (NPK and double amount of rice straw), NPKPM (NPK and pig manure), and NPKGM (NPK and green manure ( Astragalus sinicus L.)) were employed with rice–rice ( Oryza sativa L.) cropping system. Rice yields, soil organic C (SOC) and total N were analyzed. In all of the treatments, early rice yields increased along cultivation years steadily, however late and annual rice yield was fluctuant between different years. From 1991 to 2006, average annual yield ranged from 7795 to 8572 kg ha −1 among different fertilizer treatments. Organic amendments usually enhanced rice yields significantly except for the treatment with NPKRS. SOC and total N contents of surface soil increased linearly with cultivated years from 3.9–5.7 g kg −1 and 0.46–0.57 g kg −1 in initial stage to 7.1–9.2 g kg −1 and 0.87–0.95 g kg −1 in 2005 respectively. Quantity and quality of input organic matter affected soil C dynamics and N balance. SOC sequestration rates were well correlated to modified organic C input, while SOC mineralization rates were related with either organic C input or SOC contents. Annual soil N accumulation was 13–18% of total input. However the net lost N, which was calculated based on fertilizer inputs, crop outputs, and annual N accumulation, was 44–49% in our study. In general, after 17 years’ cultivation and fertilization management, rice yield reached a high level equivalent to the average yield of local high productivity paddy soils, whereas SOC and total N content were still less than half of those in high productivity paddy soils in this region.

Can mineral and organic fertilization help sequestrate carbon dioxide in cropland?

The soil organic matter content represents a huge reservoir of plant nutrients and an effective safeguard against pollution; beside it can sequestrate atmospheric CO 2. Since 1966 up to now in the Southeast Po valley (Italy), the soil organic C (SOC) and total N (TN) dynamics in the 0–0.40 m soil layer under a maize–wheat rainfed rotation are studied as influenced by organic and mineral N fertilizations. Every year in the same plots cattle manure, cattle slurry, and crop residues (i.e. wheat straw and maize stalk) are ploughed under to 0.40 m depth at a same dry matter rate (6.0 and 7.5 t DM ha −1 year −1 after wheat and maize, respectively) and are compared to an unamended control. Each plot is splitted to receive four rates of mineral fertilizer (0–100–200–300 kg N ha −1). In the whole experiment, in 2000 SOC concentration was lower than in 1966 (6.77 and 7.72 g kg −1, respectively), likely for the deeper tillage that diluted SOC and favoured mineralization in deeper soil layer. From 1972 to 2000 SOC stock did not change in the control and N fertilized plots, while it increased at mean rates of 0.16, 0.18, and 0.26 t ha −1 year −1 with the incorporation of residues, slurry and manure, corresponding to sequestration efficiencies of 3.7, 3.8 and 8.1% of added C with the various materials. TN followed the same SOC dynamic, demonstrating how it depends on the soil organic matter. Manure thus confirmed its efficacy in increasing both SOC content and soil fertility on the long-term. In developed countries, however, this material has become scarcely available; slurry management is expensive and implies high environmental risks. Moreover, in a C balance at a farm (or regional) scale, the CO 2 lost during manure and slurry stocking should be considered. For these reasons, the incorporation of cereal residues, even if only a little of their C content was found capable of soil accumulation, appears the best way to obtain a significant CO 2 sequestration in developed countries. Our long-term experiment clearly shows how difficult it is to modify SOC content. Moreover, because climate and soil type can greatly influence SOC dynamic, to increase CO 2 sequestration in cropland, it is important to optimize the fertilization within an agricultural management that includes all the agronomic practices (e.g. tillage, water management, cover crops, etc.) favouring the organic matter build up in the soil.

Carbon-13 Fractionation of Relic Soil Organic Carbon during Mineralization Effects Calculated Half-Lives

The 13 C natural abundance approach for determining soil organic C (SOC) stability and turnover has been used to determine SOC mineralization kinetics. These calculations generally assume that 13 C fractionation during relic SOC and unharvested biomass mineralization is insignificant. The objective of this study was to determine the impact of this assumption on calculated relic SOC half-lives. Study sites were located in Minnesota and South Dakota. At the Minnesota site, SOC contained in the surface 30 cm of soil in a fallowed area decreased from 90.8 to 73.2 Mg ha -1 during a 22-yr period. Associated with this decrease was a 0.72%o increase in the soil δ 13 C values (from -18.97 to -18.25%o). Based on these values, the Rayleigh fractionation constant (e) of relic SOC was -3.45%o. At the South Dakota site, SOC decreased 10% (2.8 ± 1.8 g kg -1 ) and δ 13 C increased 3.2% (0.548 ± 0.332%o) during a 5-yr period. The Rayleigh fractionation constant for this experiment was -6.94%o (±4.74‰). In a separate experiment, the δ 13 C value of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] residue remained unchanged after 4 mo. The impact of 13 C enrichment during relic C mineralization on calculated C budgets depends on the type of residue returned to the soil. A simulation study showed that for systems where C 4 residues are returned to soil derived from C 3 and C 4 plants, not considering 13 C enrichment during relic SOC mineralization will result in underestimating relic SOC half-lives and overestimating the contribution of fresh C 4 biomass in the SOC. The effect of 13 C enrichment during relic SOC and unharvested biomass mineralization had cumulative impacts on C budgets and did not cancel each other out. The reverse was true for C 3 biomass. To minimize these errors, SOC maintenance rate experiments should measure 13 C enrichment during relic SOC and unharvested biomass mineralization.

Theoretical Derivation of Stable and Nonisotopic Approaches for Assessing Soil Organic Carbon Turnover

Techniques for measuring soil organic C (SOC) turnover in production fields are needed. The objectives of this study were to propose and test nonisotopic and 13C stable isotopic techniques for assessing SOC turnover. Based on SOC equilibrium and mass balance relationships, an equation was derived: NHC/SOCinitial = [1/(SOC × kNHC)](dSOC/dt) + kSOC/kNHC, where dSOC/dt is the annual change in SOC, NHC is nonharvested C returned to soil, kSOC is the annual mineralization rate of SOC, and kNHC is the annual mineralization rate of NHC. This equation was used to calculate maintenance rates. An isotopic approach based on simultaneously solving the equations was developed to determine C budgets: (i) SOCretained = [SOCfinal (Δsoil final − ΔPCR)/(ΔSOCretained − ΔPCR)]; (ii) ΔSOC retained = Δsoc initial − [ε ln(SOCretained/SOCinitial)], (iii) ΔPCR = ΔNHC − [ε ln(PCR/NHC)]; and (iv) SOCfinal = SOCretained + PCRincorp, where ε is the Rayleigh fractionation constant, PCRincorp is the amount of NHC incorporated into SOCfinal with ΔPCR being the associated 13C discrimination (Δ) value, and SOCretained is the amount of SOCinitial retained in the soil after mineralization with ΔSOC retained being the associated Δ value. Isotopic and nonisotopic approaches were tested on a production field where aboveground corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields were measured with a yield monitor and soil samples collected from a slightly offset grid (30 m) in 1995 and 2003 were analyzed for organic C and Δ. The nonisotopic approach showed that maintenance rates increased with SOC and that an accurate measure of NHC was required to calculate maintenance requirements. Sensitivity analysis of the isotopic approach showed that calculated budgets were sensitive to 13C discrimination during SOC mineralization. If 13C discrimination during SOC and NHC mineralization did not occur (ε = 0), then 14.9 and 7.6% of the SOC measured in 1995 (SOCinitial) was mineralized, and 7420 and 2780 kg C ha−1 of NHC were incorporated into SOCfinal in the 523.4‐ to 527.3‐ and 527.3‐ to 529.2‐m elevation zones, respectively. If 13C discrimination occurred (ε = −2.52‰) during SOC mineralization, then the calculated amount of SOC mineralized and the amount of new C incorporated into SOC were reduced.