Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

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Climate Change 2007: The Physical Science Basis.

Climate change 2014 - Synthesis Report

Changes in agricultural management can potentially increase the accumulation rate of soil organic C (SOC), thereby sequestering CO 2 from the atmosphere. This study was conducted to quantify potential soil C sequestration rates for different crops in response to decreasing tillage intensity or enhancing rotation complexity, and to estimate the duration of time over which sequestration may occur. Analyses of C sequestration rates were completed using a global database of 67 long-term agricultural experiments, consisting of 276 paired treatments. Results indicate, on average, that a change from conventional tillage (CT) to no-till (NT) can sequester 57 ± 14 g C m -2 yr -1 , excluding wheat (Triticum aestivum L.)-fallow systems which may not result in SOC accumulation with a change from CT to NT. Enhancing rotation complexity can sequester an average 20 ± 12 g C m -2 yr -1 , excluding a change from continuous corn (Zea mays L.) to corn-soybean (Glycine max L.) which may not result in a significant accumulation of SOC. Carbon sequestration rates, with a change from CT to NT, can be expected to peak in 5 to 10 yr with SOC reaching a new equilibrium in 15 to 20 yr. Following initiation of an enhancement in rotation complexity, SOC may reach a new equilibrium in approximately 40 to 60 yr. Carbon sequestration rates, estimated for a number of individual crops and crop rotations in this study, can be used in spatial modeling analyses to more accurately predict regional, national, and global C sequestration potentials.

Soil organic carbon sequestration rates by tillage and crop rotation : A global data analysis

Although it is well established that soils are the dominating source for atmospheric nitrous oxide (N2O), we are still struggling to fully understand the complexity of the underlying microbial production and consumption processes and the links to biotic (e.g. inter- and intraspecies competition, food webs, plant–microbe interaction) and abiotic (e.g. soil climate, physics and chemistry) factors. Recent work shows that a better understanding of the composition and diversity of the microbial community across a variety of soils in different climates and under different land use, as well as plant–microbe interactions in the rhizosphere, may provide a key to better understand the variability of N2O fluxes at the soil–atmosphere interface. Moreover, recent insights into the regulation of the reduction of N2O to dinitrogen (N2) have increased our understanding of N2O exchange. This improved process understanding, building on the increased use of isotope tracing techniques and metagenomics, needs to go along with improvements in measurement techniques for N2O (and N2) emission in order to obtain robust field and laboratory datasets for different ecosystem types. Advances in both fields are currently used to improve process descriptions in biogeochemical models, which may eventually be used not only to test our current process understanding from the microsite to the field level, but also used as tools for up-scaling emissions to landscapes and regions and to explore feedbacks of soil N2O emissions to changes in environmental conditions, land management and land use.

https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.2013.0122

Nitrous oxide emissions from soils: how well do we understand the processes and their controls?

Extracellular enzymes are the proximate agents of organic matter decomposition and measures of these activities can be used as indicators of microbial nutrient demand. We conducted a global-scale meta-analysis of the seven-most widely measured soil enzyme activities, using data from 40 ecosystems. The activities of b-1,4-glucosidase, cellobiohydrolase, b-1,4-N-acetylglucosaminidase and phosphatase g )1 soil increased with organic matter concentration; leucine aminopeptidase, phenol oxidase and peroxidase activities showed no relationship. All activities were significantly related to soil pH. Specific activities, i.e. activity g )1 soil organic matter, also varied in relation to soil pH for all enzymes. Relationships with mean annual temperature (MAT) and precipitation (MAP) were generally weak. For hydrolases, ratios of specific C, N and P acquisition activities converged on 1 : 1 : 1 but across ecosystems, the ratio of C : P acquisition was inversely related to MAP and MAT while the ratio of C : N acquisition increased with MAP. Oxidative activities were more variable than hydrolytic activities and increased with soil pH. Our analyses indicate that the enzymatic potential for hydrolyzing the labile components of soil organic matter is tied to substrate availability, soil pH and the stoichiometry of microbial nutrient demand. The enzymatic potential for oxidizing the recalcitrant fractions of soil organic material, which is a proximate control on soil organic matter accumulation, is most strongly related to soil pH. These trends provide insight into the biogeochemical processes that create global patterns in ecological stoichiometry and organic matter storage.

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Stoichiometry of soil enzyme activity at global scale

Nitrous oxide (N2O) is a potent greenhouse gas, which is mainly produced from agricultural soils. Ammonia oxidation is the rate-determining step in N2O production, and the process is carried out by ammonia oxidizers, bacteria and archaea. Soil aggregate size has been shown to alter soil properties, which affect N2O emissions and bacterial communities. However, the effect of aggregate size on temporal and total N2O emissions and ammonia-oxidizing bacteria (AOB) and archaea (AOA) is not fully understood. This incubation study investigated the effect of three different soil aggregate sizes on N2O emissions and ammonia oxidizer abundance under high urine-N concentrations and the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), at reducing N2O emissions in different aggregate soils. It was found that temporal patterns of N2O emissions were affected by aggregate size with higher peak emissions in the large and medium aggregates. However, the total emissions were the same due to a ‘switch’ in emissions at day 66, after which smaller aggregates produced higher N2O emissions. It is suggested that the switch was caused by an increase in aggregate disruption in the small aggregates, following the urine application, due to their higher surface area to volume ratio. AOB and AOA abundances were not significantly affected by aggregate size. DCD was effective in reducing N2O emissions in all aggregate sizes by an average of 79%. These results suggest that similar ammonia oxidizer abundance is found in soils of different aggregate sizes, and the efficacy of DCD in reducing N2O emissions was not affected by aggregate size of the soil.

Effect of soil aggregate size and dicyandiamide on N2O emissions and ammonia oxidizer abundance in a grazed pasture soil

Winter forage grazing systems in New Zealand cause compaction of soil by grazing animals, especially when the soil is wet. However, there is little information on the effects of animal trampling on denitrifiers in soil, despite their importance for N2O production. Here, we report a field study of the abundance of the denitrifying genes nirS, nirK, and nosZ and N2O emissions following the application of dairy cow urine in a free-draining stony soil. Importantly, we found that simulated animal trampling altered some of the denitrifying microbial communities, thus leading to increased N2O emissions. Over the 111 day measurement period, the abundance of nitrite (NO2−)-reducing nirS gene copy numbers increased significantly by 87% in the trampled soil with urine (P < 0.01) and increased by 40% in the trampled soil without urine (P < 0.05), but the nirS gene abundance did not change significantly in the nontrampled soil. The abundance of NO2− reducing nirK gene copy numbers was not affected by trampling, but increased significantly following urine application. The abundance of N2O-reducing nosZ clade I and nosZ clade II gene copy numbers increased significantly in the trampled soil, but did not change significantly in the nontrampled soil. N2O emissions from the trampled soil were about twice that from the nontrampled soil without urine (1.20 and 0.62 kg N2O-N per ha, respectively) and about eight times greater (6.24 kg N2O-N per ha) than from nontrampled soil (0.80 kg N2O-N per ha) when urine was applied. These results strongly suggest that animal trampling during winter forage grazing can have a major impact on denitrifying communities in soil, which in turn stimulate greater denitrification with increased N2O emissions.

Simulated animal trampling of a free‐draining stony soil stimulated denitrifier growth and increased nitrous oxide emissions

Porosity: Pore Size Distribution

The effects of pre-tillage soil water content (PTSW) and multiple-pass tillage on soil physical condition was investigated on a silt-loam soil susceptible to wind erosion. PTSW was shown to affect the soil condition produced by tillage and an interaction between intensity of tillage and PTSW was identified. The ‘excess’ tillage treatment produced 13.9% of aggregates Aggregates produced from ‘excessive’ tillage were more stable than those from the ‘intermediate’ and ‘minimum’ tillage treatments when dry aggregate stability was determined soon after tillage. Aggregates formed during tillage with PTSW near the plastic limit were less resistant to mechanical breakdown when compared with those formed at lower PTSW levels. Excessive tillage at the lower plastic limit and at 0.58 times the lower plastic limit resulted in dry aggregate stability indices of 0.835 and 0.923, respectively (significant). More intense tillage produced a smoother soil surface, which increases the susceptibility of the soil to wind erosion. Random soil roughness and soil surface area were not affected by PTSW. Dry bulk density was lower for ‘minimum’ tillage as compared with the other two treatments. Excessive tillage of this soil should be avoided, especially at low PTSW, because of the large quantities of wind-erodable aggregates which can result. In assessing the most appropriate PTSW for desired tillage objectives, the effect on aggregate stability must be considered.

Tillage and water content effects on surface soil physical properties

This research was
funded by the Lincoln University Centre for Soil and
Environmental Research.

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Keith C. Cameron

Papers

Effect of soil aggregate size and dicyandiamide on N2O emissions and ammonia oxidizer abundance in a grazed pasture soil

Simulated animal trampling of a free‐draining stony soil stimulated denitrifier growth and increased nitrous oxide emissions

Porosity: Pore Size Distribution

Tillage and water content effects on surface soil physical properties

The effect of DCD on nitrogen losses from sheep urine patches applied to lysimeters in autumn.