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

Most farmers are highly conscious of the importance of sustainable land management. However, the economics of present day agriculture place ever increasing pressure on farmers to intensify land-use, often exceeding the margins of what is truly sustainable. As a result, farmers increasingly demand more information and better tools for monitoring the sustainability of their agricultural management. This paper describes work in progress. It is our intent to present an overview of our research and development of a Soil Quality Monitoring System (SQMS). The SQMS is designed as an on-farm tool for monitoring and interpreting changes in soil quality that reflect on the sustainability of agricultural management practices. The paper focuses on the philosophical and theoretical basis of our research, the specific approach we have taken and some future prospects for the SQMS.

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Soil quality monitoring for sustainable agriculture

Nitrous oxide (N2O) emissions from intensively grazed pasture systems contribute significantly to total greenhouse gases. A series of field-plot experiments were conducted at three geographically separated sites to study the ammonia oxidising microbial populations and their relationships with N2O emissions. Our results support the findings from earlier incubation experiments, showing that growth of ammonia oxidising bacteria (AOB), and not ammonia oxidising archaea (AOA), was stimulated by the high concentration of urine. There was substantial spatial variability of the AOB and AOA amoA gene copy numbers at different sites and within replicate samples. AOB abundance was generally higher in the top 2.5 cm compared with the top 10 cm of soil, whereas the opposite trend was observed for AOA. Positive relationships were found between the N2O emissions and AOB abundance, although the relationship varied at the three sites.

https://www.tandfonline.com/doi/pdf/10.1080/00288233.2014.924969

Ammonia oxidising populations and relationships with N2O emissions in three New Zealand soils

The aim of this research was to quantify the effect of plantain (Plantago lanceolata L.) on soil nitrification rate, functional gene abundance of soil ammonia oxidisers, and the concomitant effect on nitrous oxide emissions from urine patches in a shallow, free-draining soil in Canterbury during late autumn/winter season. Urine was collected from dairy cows grazing either ryegrass/white clover (RGWC), 30% plantain (P30) mixed in with RGWC or 100% plantain (P100) pasture, and applied at two rates (700 or 450 kg N ha−1) to intact soil blocks growing either RGWC, P30 or P100 pasture. Results showed that increased plantain content reduced N-concentration in urine from 7.2 in RGWC urine to 4.5 and 3.7 g N L−1 in P30 and P100 urine, respectively. Total N2O emissions and emission factors (EF3) from urine-treated pastures were low, < 2 kg N ha−1 and < 0.22%, respectively. Urine application at the lower urine N-loading rate of 450 kg N ha−1 (i.e. representative of that in a P30 urine patch) resulted in 30% lower N2O emissions (P < 0.01) and 35% lower soil nitrate concentrations (P < 0.001) compared to those at the higher urine loading rate of 700 kg N ha−1 (i.e. representative of that in a RGWC urine patch). Increasing plantain content in the pasture sward from 0 to 30% and 100% with urine N applied at the same loading rate did not reduce N2O emissions or nitrification compared to the standard ryegrass-white clover pasture. Cow urine derived from the different pasture diets had no effect on N2O emissions, N transformation or ammonia-oxidiser abundance in soil compared to the RGWC urine applied at the same rate. The main effect of plantain in this study appears to be related to the reduction in urine N-loading rate, rather than factors related to urine properties or plantain-soil interactions.

Effect of plantain on nitrous oxide emissions and soil nitrification rate in pasture soil under a simulated urine patch in Canterbury, New Zealand

Effects of lime and phosphate additions plus subsequent leaching on cation exchange capacities of soils with variable charge

15Nitrogen balances are useful tools to increase our understanding of soil N processes and the efficiency of plants in taking up animal urine-N. This study aimed to investigate the effect of forage type on the fate of autumn-applied urine-N (700 kg N ha−1). In a 17-month lysimeter study the recoveries of 15N in herbage, leachate, nitrous oxide, and soil + roots (at the end of the trial) were determined for three forage types: perennial ryegrass-white clover (RGWC, Lolium perenne L.-Trifolium repens L.), Italian ryegrass (Italian RG, Lolium multiflorum Lam.), and lucerne (Medicago sativa L.) under simulated autumn grazing. We recovered 81.5–85.5% of the 15N applied in the urine. Italian RG had the greatest amount of urine-15N recovered in the herbage (49.5%) and the least recovered in leachate (16.8%) making it the most efficient of the three forages for utilizing urine-N. For RGWC, herbage and leachate recoveries were 40.1% and 23.7%, respectively. Lucerne was the least efficient with recoveries of 15N in herbage and leachate equaling 18.4% and 52.5%, respectively. This study demonstrates that Italian ryegrass could offer benefits in reducing urine-N leaching losses through its ability to take up more urine-N over winter.

Keith C. Cameron

Papers

Soil quality monitoring for sustainable agriculture

Ammonia oxidising populations and relationships with N2O emissions in three New Zealand soils

Effect of plantain on nitrous oxide emissions and soil nitrification rate in pasture soil under a simulated urine patch in Canterbury, New Zealand

Effects of lime and phosphate additions plus subsequent leaching on cation exchange capacities of soils with variable charge

15N recoveries from ruminant urine patches on three forage types