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Doscover Magazine

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Dr. Dick van der Wateren

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Greenhouse Gases, Field Measurements, Climate Change, Agricultural Ecosystems, Carbon Sequestration, Agriculture


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Changing farming practices help reverse global warming

03.07.2011, Age: 2717 days

Using compost, keeping fields planted year-round, reducing tillage and increasing plant diversity helps to reduce carbon loss to the atmosphere. New research has led to several good farming practices that have the combined effect of increasing food production in poor countries and reducing atmospheric carbon dioxide.

It has long been known that fertile dark and moist soils have a high capacity for retaining carbon, while soils that lay barren rapidly lose carbon to the atmosphere where it contributes to greenhouse warming. Now, this effect has been quantified by professor Whendee Silver, University of California, Berkeley, in a collaboration with ranchers and land management organisations.

Her research is part of the Marin Carbon Project:

"Reducing greenhouse gas emissions is not enough to reverse global warming: we must also reduce the concentration of carbon dioxide in the atmosphere. The Marin Carbon Project is investigating the potential for specific land management practices to enhance sequestration of atmospheric carbon dioxide as organic matter in rangeland and agricultural soils in California."

One of the conclusions of this project is that fertilising soils with compost enhances vegetation growth. Silver and her co-workers calculate that

"28 million acres of grazing land in California could absorb 42 million tons of carbon dioxide-nearly 40 percent of what the state's electrical power plants produce in a year. To accomplish that, each acre of land must absorb just 1.5 additional tons of carbon dioxide."

Ryals, R.; Silver, W. L. 2010. Effects of management of ecosystem carbon pools and fluxes in grassland ecosystems. American Geophysical Union, Fall Meeting 2010, abstract #B14A-04.

Abstract

Grasslands represent a large land-use footprint and have considerable potential to sequester carbon (C) in soil. Climate policies and C markets may provide incentives for land managers to pursue strategies that optimize soil C storage, yet we lack robust understanding of C sequestration in grasslands. Previous research has shown that management approaches such as organic amendments or vertical subsoiling can lead to larger soil C pools. These management approaches can both directly and indirectly affect soil C pools.

We used well-replicated field experiments to explore the effects of these management strategies on ecosystem C pools and fluxes in two bioclimatic regions of California (Sierra Foothills Research and Extension Center (SFREC) and Nicasio Ranch). Our treatments included an untreated control, compost amendments, plowed (vertical subsoil), and compost + plow. The experiment was conducted over two years allowing us to compare dry (360 mm) and average (632 mm) rainfall conditions. Carbon dioxide (CO2) fluxes were measured weekly using a LI-8100 infrared gas analyzer. Methane (CH4) and nitrous oxide (N2O) fluxes were measured monthly using static flux chambers. Aboveground and belowground biomass were measured at the end of the growing season as an index of net primary productivity (NPP) in the annual plant dominated system. Soil moisture and temperature were measured continuously and averaged on hourly and daily timescales. Soil organic C and N concentrations were measured prior to the application of management treatments and at the ends of each growing season.

Soils were collected to a 10 cm depth in year one and at four depth increments (0-10, 10-30, 30-50, and 50-100 cm) in year two. Soil C and N concentrations were converted to content using bulk density values for each plot. During both growing seasons, soil respiration rates were higher in the composted plots and lower in the plowed plots relative to controls at both sites.

The effects on C loss via soil respiration were stronger in the first year, with compost soils experiencing a 21 ± 1 % greater cumulative loss at SFREC and 16 ± 3 % more at Nicasio. The second year showed a similar trend, but with a lower magnitude loss. Aboveground NPP responded positively to compost additions and negatively to plowing at both sites. At SFREC, we measured 58 % more ANPP in composted relative to control plots in year one (369 vs 230 g C/m2) and 56 % more in year two (327 vs 209 g C/m2). Aboveground NPP on plowed plots was 129 g C/m2 in year one, and 185 g C/m2 in year two. Plowed soils also showed a significant decline in soil C and N concentrations (C= 2.67 ± 0.13%, N = 0.20 ± 0.01%). Compost additions increased soil C and N concentrations (C= 3.92 ± 0.29%, N = 0.32 ± 0.02%) relative to control soils (C= 3.52 ± 0.20%, N = 0.27 ± 0.07%).

Throughout the experiment, we did not detect significant treatment differences in CH4 or N2O fluxes, nor did we detect significant differences at any individual sampling point. These results suggest that compost addition can lead to an increase in ecosystem C storage, with a small offset from elevated soil respiration.


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