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Reducing CH4and N2O emissions from rice cropping systems while sustaining production levels with less water requires a better understanding of the key processes involved. Alternate wetting and drying (AWD) irrigation is one promising practice that has been shown to reduce CH4emissions. However, little is known about the impact of this practice on N2O emissions, in particular under Mediterranean climate. To close this knowledge gap, we assessed how AWD influenced grain yield, fluxes and annual budgets of CH4and N2O emissions, and global warming potential (GWP) in Italian rice systems over a 2-year period. Overall, a larger GWP was observed under AWD, as a result of high N2O emissions which offset reductions in CH4emissions. In the first year, with 70% water reduction, the yields were reduced by 33%, CH4emissions decreased by 97%, while N2O emissions increased by more than 5-fold under AWD as compared to PF; in the second year, with a 40% water saving, the reductions of rice yields and CH4emissions (13% and 11%, respectively) were not significant, but N2O fluxes more than doubled. The transition from anaerobic to aerobic soil conditions resulted in the highest N2O fluxes under AWD. The duration of flooding, transition to aerobic conditions, water level above the soil surface, and the relative timing between fertilization and flooding were the main drivers affecting greenhouse gas mitigation potential under AWD and should be carefully planned through site-specific management options.

AbstractIn permafrost-affected sites, soil forming processes appear to be closely connected with organic matter (OM) accumulation. In this work OM composition and nutrient availability has been evaluated in a frost-affected soil located at 1200 m a.s.l. in Creux du Van (Switzerland), where patches of stunted Norway spruce trees adjacent to tall trees have been ascribed to the presence of sporadic alpine permafrost. Soil samples were collected under the stunted forest and in the adjacent tall forest and characterized for their chemical and physical characteristics. The main C and N forms have been determined and characterized.Under the stunted forest the soil samples showed a high total organic C/total N (TOC/TN) ratio and scarce microbial activity; humification processes were limited and humic acids revealed little oxidation, scarce incorporation of N-containing moieties, and high enrichment of lipids. 14C dating revealed the presence in the bulk samples of young organic material mixed with relatively old humic acids, probably due to cryoturbation processes. These latter processes appeared also responsible for the arrival of fresh litter material from Oi and Oe horizons into the Oa horizon of the stunted forest and for the consequent genesis of humic substances from a mix of old and fresh residues.From our findings there seems to exist a reciprocal influence of vegetation quality on the OM composition, and of OM decomposition on nutrition, as driven by the microclimatic conditions and physical processes, which in turn may contribute to keep the soil at an unstable developmental stage and limit the spruce growth.

Abstract The carbon dioxide efflux to the atmosphere and the concentrations at various depths in two soils were measured, for more than a year, under pure stands of silver fir ( Abies alba Mill.) and European beech ( Fagus sylvatica L.) in central Italy. Microbial biomass and activity at the monitored depths were determined in the laboratory and the CO₂ evolved from incubated samples was submitted to radiocarbon analysis to assess the mean residence time of the organic matter degraded by microorganisms. The CO₂ efflux showed similar trends in the two soils, with highest values in October and lowest in January. The efflux depended more on air and soil temperatures than soil moisture, and was related to these variables better under fir than under beech. In both soils, the CO₂ concentration increased with depth: in the top horizon it was low and similar to that of the atmosphere, while in the deeper horizons it often amounted to considerable values (up to more than 1% by volume in the BC horizon under fir). The subsoil of the fir stand generally showed much higher CO₂ concentrations than that of the beech. The basal respiration as determined in the laboratory was at a maximum in the topsoil and decreased sharply downwards. Therefore, the high CO₂ concentrations measured in the field at the bottom of the profiles--where roots were few, and microbial biomass and available C pool were at a minimum--appeared to be due more to slow diffusivity of the soil matrix rather than to heavy release of the gas by the biota. The organic matter respired by microorganisms in incubated soil samples showed positive values of Δ¹⁴C that revealed a recent synthesis. The estimated mean residence time increased with depth, suggesting a generally higher degree of stabilisation of the organic pool in the subsoil.

Although much is known about the transformation of humic substances in the fine earth fraction of surface horizons, little is known about these processes in the coarse fragments (>2 mm) or about the changes with depth of humic and fulvic acids extracted from an entire soil profile. For each horizon of a forest soil developed on sandstone, the humic acids (HAs) and fulvic acids (FAs) were extracted from four fractions fine earth, washing (i.e. fine material adhering to the clasts) and highly altered and slightly altered rock fragments. The HAs and FAs were also extracted from samples of the parent rock. The humic substances were then analyzed for their elemental composition and chemical structure by FT-IR and 13C NMR spectroscopy. In the A horizons (upper 20 cm of the profile), the HAs from all the fractions appeared highly aromatic, with a high degree of C substitution and an abundance of carboxyl groups; however, the HAs of the rock fragments were more aromatic and richer in carbohydrate and proteinaceous residues than those of the fine earth. The HAs showed a more condensed and unsubstituted aromatic pattern at depth than in surface horizons. In the Bw and BCb horizons the contribution of biological residues to the humic substances seems to be progressively replaced by the contribution of organic materials derived from the sedimentary parent rock. The presence of features ascribable to the HAs and FAs of the parent rock decreased following the order slightly altered clasts > highly altered clasts > washing > fine earth. Conversely, FAs showed a high homogeneity throughout the profile and are considered mostly of pedogenic origin. The chemical characteristics of both HAs and FAs showed a break in the trend with depth between Bw and BCb horizons, confirming the presence of a pedogenic discontinuity between the upper modern soil (A-Bw2 horizons) and the buried paleosol (BCb horizons).

Rock fragments (particles > 2 mm) are usually considered chemically inert for plant growth. In this paper, the potential fertility in terms of exchangeable Ca, Mg, and K of rock fragments from sandstone and siltstone derived soils from northern Apennines (Italy) is reported and contrasted with that of the fine earth (particles < 2 mm). The results show that rock fragments are a source of Ca, Mg, and K. When expressed on a volume basis, the abundance of these exchangeable nutrients sometimes may equal or surpass that of the fine earth. The plant uptake of Mg and K has been demonstrated in growth experiments with Agrostis under controlled conditions.