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

The effect of Ca(OH)2 or Ca(H2PO4)2 ⋅ 2H2O (MCP) on potassium (K) or magnesium (Mg) leaching through and out of columns of soil with predominantly variable charge was studied. Calcium hydroxide was mixed with soil from the A and B horizon to raise the pH to about 6 or 7, and MCP, equivalent to 952 mg P, was mixed with the A horizon of each soil. Various concentrations of KCl or MgCl2 were applied as a pulse to the soil surface and leached with five pore volumes of deionised water.

Effect of cation exchange on the distribution and movement of cations in soils with variable charge. II. Effect of lime or phosphate on potassium and magnesium leaching

Dairy winter forage grazing on free-draining soils is a common practice within the Canterbury region of New Zealand. The high stocking rates involved and the associated deposition of urine onto wet soils during winter create a high risk of nitrate (NO3
 −) leaching from soil. The objective of this study was to determine the effect of animal trampling and the use of a nitrification inhibitor, dicyandiamide (DCD), on soil ammonia oxidisers, nitrification, and nitrate leaching under simulated winter forage grazing conditions. A lysimeter trial was carried out using a Balmoral stony silt loam under a kale forage crop. Nitrate leaching losses, the effect of soil trampling, and the effect of DCD were measured. Soil nitrification rates and ammonia-oxidising community abundance and activity were measured in companion soil blocks under simulated dairy winter forage grazing conditions. Animal trampling was found to reduce peak nitrate-N (NO3
 --N) concentrations in drainage water from urine patch areas by 43 %. In addition, animal trampling reduced the total amount of NO3
 −-N leached from urine patches by 34 %. However, animal trampling did not affect the growth or activity of the ammonia oxidisers. The use of DCD was found to be highly effective in reducing the concentration and amount of NO3
 −-N leached from urine patches. Dicyandiamide applications reduced peak NO3
 −-N concentrations in drainage water by 66 %, and the total amount of NO3
 −-N leached was reduced by 61 % under the simulated dairy winter forage grazing conditions. Ammonia oxidising bacteria (AOB) were more abundant than ammonia-oxidising archaea (AOA) and were responsible for mediating the nitrification process. These results suggest that both animal trampling and the use of DCD separately reduces soil nitrification rates and thus NO3
 − leaching.

The effect of animal trampling and DCD on ammonia oxidisers, nitrification, and nitrate leaching under simulated winter forage grazing conditions

The influence of Ca(OH)2 on K and Mg exchange equilibria in three New Zealand soils was studied. Calcium hydroxide was mixed with each soil to raise the pH to about 6 or 7. For each Ca(OH)2 treatment, K and Mg exchange isotherms were determined, from which the equilibrium activity ratios were derived. Exchange coefficients and solution activity ratios were calculated according to the Gapon convention.

Effect of cation exchange on the distribution and movement of cations in soils with variable charge. I. Effect of lime on potassium and magnesium exchange equilibria

Three forms of sulfur (S) were determined in extracts from field-moist, air-dried and conditioned (i.e. maintained at 20°C and 75% field moisture capacity for 2 weeks) soils by using four extractants-water, CaCl2, KH2PO4 and Ca(H2PO4)2. The amounts and proportions of inorganic sulfate (SO2-4-S), organic HI-reducible S (HI-S) and carbon bonded S (C-S) in extracts varied with soil type, method of sample preparation and the extractant used. Substantially less S overall was extracted from field-moist soil compared with air-dried and conditioned soils. For soils with pH values close to or above 6, water and phosphate extractants removed similar amounts of SO2-4-S. However, phosphate solutions removed greater amounts of SO2-4-S than water from soils with relatively low pH or with a large capacity for sulfate adsorption. The CaCl2 solution extracted the smallest amounts of sulfate from the soils studied. There was no consistent ratio between extractable sulfate-S and extractable organic S. Extracts from air-dried soil contained the greatest amounts and proportions of organic S, particularly in phosphate extracts. The conditioning of soils decreased the extractable organic S and increased the sulfate-S, indicating that extractable organic S is readily mineralizable. The relative proportions of extractable organic S present as HI-S and C-S varied considerably. Comparison of the conditioned and air-dried soils indicated there was no difference between the two forms of organic S in their ease of mineralization.

Forms of sulfur extracted from soils after different methods of sample preparation

Methanotrophs are an important group of methane (CH4)-oxidizing bacteria in the soil, which act as a major sink for the greenhouse gas, CH4. In grazed grassland, one of the ecologically most sensitive areas is the animal urine patch soil, which is a major source of both nitrate (NO3
 −) leaching and nitrous oxide (N2O) emissions. Nitrification inhibitors, such as dicyandiamide (DCD), have been used to mitigate NO3
 − leaching and N2O emissions in grazed pastures. However, it is not clear if the high nitrogen loading rate in the animal urine patch soil and the use of nitrification inhibitors would have an impact on the abundance of methanotrophs in grazed grassland soils. The purpose of this study was to determine the effect of animal urine and DCD on methanotroph abundance in grazed grassland soils. A laboratory incubation study was conducted to determine the effect of urine and DCD applications on the abundance of methanotrophs in six grazed grassland soils sampled from across New Zealand, using real-time PCR targeting the functional pmoA gene. Results showed that the pmoA gene copy numbers were low in these soils, mostly below 2.36 × 104 g−1 soil except in the West Coast soil where pmoA gene copy number reached 8.95 × 105 g−1 soil. Most of the clones identified were aligned to the type II methanotrophs. There was no significant effect (P < 0.05) on the abundance of methanotrophs by the applications of urine at 1,000 kg N ha−1 or DCD at 10 kg ha−1. These results suggest that the abundance of methanotrophs is not affected by urine deposition or the application of DCD to mitigate NO3
 − leaching and N2O emissions in grazed grassland soils.

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Keith C. Cameron

Papers

Effect of cation exchange on the distribution and movement of cations in soils with variable charge. II. Effect of lime or phosphate on potassium and magnesium leaching

The effect of animal trampling and DCD on ammonia oxidisers, nitrification, and nitrate leaching under simulated winter forage grazing conditions

Effect of cation exchange on the distribution and movement of cations in soils with variable charge. I. Effect of lime on potassium and magnesium exchange equilibria

Forms of sulfur extracted from soils after different methods of sample preparation

Methanotroph abundance not affected by applications of animal urine and a nitrification inhibitor, dicyandiamide, in six grazed grassland soils