Cultivation effects on temporal changes of organic carbon and aggregate stability in desert soils of Hexi Corridor region in China

Sustainable agricultural use of cultivated desert soils has become a concern in Hexi Corridor in Gansu Province of China, because loss of topsoil in dust storms has been recently intensified. We chose four desert sites to investigate the effects of cultivation (cropping) on (i) soil organic C and its size fractions and (ii) soil aggregate stability (as a measure of soil erodibility). These parameters are of vital importance for evaluating the sustainability of agricultural practices.

Total organic C as well as organic C fractions in soil (coarse organic C, 0.1–2 mm; young organic C, 0.05–0.1 mm; stable organic C, <0.05 mm) generally increased with the duration of the cultivation period from 0 (virgin soil, non-cultivated) to more than 30 years (p < 0.05). Compared to total organic C in virgin soils (2.3–3.5 g kg⁻¹ soil), significantly greater values were found after 10 to >20 years of cultivation (6.2–7.1 g kg⁻¹ soil). The increase in organic C in desert soils following prolonged cultivation was mainly the consequence of an increase in the coarse organic C. The increase in total organic C in soil was also dependent on clay content [total organic C = 0.96 + 0.249 clay content (%) + 0.05 cultivation year, R² = 0.48, n = 27, p < 0.001]. This indicates that clay protected soil organic C from mineralization, and also contributed to the increase in soil organic C as time of cultivation increased.

There was a significant positive correlation between aggregate stability and total organic C across all field sites. The water stability of aggregates was low (with water-stable aggregate percentage 4% of dry-sieved aggregates of size 1–5 mm). There was no consistent pattern of increase in the soil aggregate stability with time of cultivation at different locations, suggesting that desert soils might remain prone to wind erosion even after 50 years of cultivation. Alternative management options, such as retaining harvested crop residues on soil surface and excluding or minimizing tillage, may permit sustainable agricultural use of desert soils.     

Partitioning of soil respiration into its autotrophic and heterotrophic components by means of tree-girdling in old boreal spruce forest

Forests accumulate much less carbon than the amount fixed through photosynthesis because of an almost equally large opposing flux of CO₂ from the ecosystem. Most of the return flux to the atmosphere is through soil respiration, which has two major sources, one heterotrophic (organisms decomposing organic matter) and one autotrophic (roots, mycorrhizal fungi and other root-associated microbes dependent on recent photosynthate). We used tree-girdling to stop the flow of photosynthate to the belowground system, hence, blocking autotrophic soil activity in a 120-yr-old boreal Picea abies forest. We found that at the end of the summer, two months after girdling, the treatment had reduced soil respiration by up to 53%. This figure adds to a growing body of evidence indicating (t-test, d.f. = 7, p < 0.05) that autotrophic respiration may contribute more to total soil respiration in boreal (mean 53 ± 2%) as compared to temperate forests (mean 44 ± 3%). Our data also suggests that there is a seasonal hysteresis in the response of total soil respiration to changes in temperature. We propose that this reflects seasonality in the tree below-ground carbon allocation.     

Decomposition of maize straw in saline soil

The interactive effects of salinity and water on organic matter decomposition in soil are poorly known. A loamy topsoil adjusted to five concentrations of salinity (0, 31, 62, 93 and 124 mmol Na kg⁻¹ soil) using either NaCl or Na₂SO₄ was incubated at a water content of either 17 or 25% (w/w) in the dark at 28.5°C for 47 days, with maize straw added at 20 g kg⁻¹ soil. Comparing with non-saline soil, (1) NaCl salinity at all levels decreased cumulative CO₂ evolved during days 1–3 (averaged across two water levels), increased in the period 4–32 days at both water contents, and thereafter caused variable effects, depending upon water content and salinity; and (2) Na₂SO₄ salinity at various levels mainly caused no effect on cumulative CO₂ evolved during days 1–3 (averaged across two water levels), and thereafter (i.e. in days 4–47) caused mainly positive effects at 17% (w/w) water content and negative effects at 25% (w/w) water content. Cumulative CO₂ evolved over 47 days for both types of salinities was mainly greater at 17% (w/w) and smaller at 25% (w/w) water content compared with non-saline soil. Generally, at 25% (w/w) than at 17% (w/w) water content, there was a greater CO₂ evolved over 47 days, and also during different incubation phases for both types of salinities; the difference at low salinity levels was generally large and decreased as salinity increased. In conclusion, the salinity effect depends on soil water content and incubation period or decomposition phase.     

A quantitative size–density separation method to recover and characterise decomposing crop residues added to soil

Plant residues placed in soil in mesh bags do not mimic realistic decomposition conditions. Alternative techniques, e.g. combined particle size and density methods, have been proposed to monitor in situ decomposition of plant residues, but are not appropriately evaluated for their ability to quantitatively recover and characterise decomposing residues of a range of sizes mixed with soil over long incubation times. For this study, we used canola residues of three different starting sizes (<1, 5–7 and 20–25 mm) mixed in a soil and incubated for 6 months. The modified method presented here relied on initial size separation of soil and organic particles into fractions of sizes <106 and >106-μm by wet sieving, and then by repeated floatation and decantation, using water as a density agent, to recover and characterise decomposing canola residues from a mixture of >106-μm mineral plus organic materials into >106-to-500 and >500-μm fractions. On day 0, across the three residue-size treatments, the >500-μm fraction recovered 93–96% of canola residue-C, with water-soluble residue-C loss during the recovery process quantified as representing further 4–5%. The rate of loss of residue-C in the >500-μm fraction was the largest, matching well the cumulative respiration loss of residue-C. The recovery of canola residue-C in the >500-μm fraction, determined as percent of cumulative CO₂–C respired of added C, decreased to 72–76% by the end of incubation, likely due to progressive generation of finer-sized residues and microbial/faunal metabolites. The increase of N in the >500-μm fraction accounted for ca. 40% of total soil N immobilised in the residue-amended soil. The extent and patterns of changes in C, N and S contents, and C-to-N and C-to-S ratios of separated fractions were similar among all the residue treatments during decomposition, except for the ground-residue treatment. The combined size and density separation procedure can be used to study decomposition in situ of soil-mixed plant residues of different sizes that are usually found in agro-ecosystems.     

Characterization of recently 14C pulse-labelled carbon from roots by fractionation of soil organic matter

The inability of physical and chemical techniques to separate soil organic matter into fractions that have distinct turnover rates has hampered our understanding of carbon (C) and nutrient dynamics in soil. A series of soil organic matter fractionation techniques (chemical and physical) were evaluated for their ability to distinguish a potentially labile C pool, that is ‘recent’ root and root‐derived soil C. ‘Recent’ root and root‐derived C was operationally defined as root and soil C labelled by ¹⁴CO₂ pulse labelling of rye grass–clover pasture growing on undisturbed cores of soil. Most (50–94%) of total soil + root ¹⁴C activity was recovered in roots. Sequential extraction of the soil + roots with resin, 0.1 m NaOH and 1 m NaOH allocated ‘recent’ soil + root ¹⁴C to all fractions including the alkali‐insoluble residual fraction. Approximately 50% was measured in the alkali‐insoluble residue but specific activity was greater in the resin and 1 m NaOH fractions. Hot 0.5 m H₂SO₄ hydrolysed 80% of the ¹⁴C in the alkali‐insoluble residue of soil + roots but this diminished specific activity by recovering much non‐¹⁴C organic matter. Pre‐alkali extraction treatment with 30% H₂O₂ and post‐alkali treatment extractions with hot 1 m HNO₃ removed organic matter with a large ¹⁴C specific activity from the alkali‐insoluble residue. Density separation failed to isolate a significant pool of ‘recent’ root‐derived ¹⁴C. The density separation of ¹⁴C‐labelled roots, and roots remixed with non‐radioactive soil, showed that the adhesion of soil particles to young ¹⁴C‐labelled roots was the likely cause of the greater proportion of ¹⁴C in the heavy fraction. Simple chemical or density fractionations of C appear unsuitable for characterizing ‘recent’ root‐derived C into fractions that can be designated labile C (short turnover time).     

Chemical fractionation to characterize changes in sulphur and carbon in soil caused by management

Classical chemical fractionation of soil sulphur (S) into HI‐reducible S and carbon‐bonded S does not separate S in soil into fractions that have differing mineralization potentials. Other techniques are needed to separate organic S into more labile and less labile fractions of biological significance, irrespective of their bonding relations. We have sequentially fractionated soil S and carbon (C) into their ionic forms released onto ion‐exchange resins and organic S and C extracted in alkali of increasing concentration. We evaluated the technique on pasture and arable soils that had received various fertilizer and cultivation treatments. Total S and C were greater in the soil of the fertilized pasture than in that of the unfertilized pastures. Continuous arable cropping decreased total soil S and C, whereas restoration to pasture caused an accumulation. Resin, 0.1 m NaOH, 1 m NaOH and residual fractions accounted for between 1–13%, 49–69%, 4–16% and 19–38% of total soil S and between 5–6%, 38–48%, 5–7% and 46–53% of total soil C, respectively. Among different S and C fractions, the size of the 0.1 m NaOH and residual fractions changed more with the change in land use and management. The 0.1 m NaOH fraction had a narrower C:S ratio (50–75:1) than did the residual fraction (96–141:1). The significant degree of change in these two fractions, caused by differences in land management, indicates that they may be useful indicators of change in ‘soil quality’.