Occasional tillage: How persistent are soil health benefits of no till?

(This first appeared in the Wyoming Livestock Roundup in June, 2015)

No-till crop production slows decomposition of crop residues, allowing redevelopment of soil aggregates and increasing the nutrient and water supplying potential of the soil. More moisture and nutrients reduce the need for fallow and often allow more diverse crop rotations. Even perennial forage production for hay and pasture benefit from minimizing or eliminating tillage when stands are renewed.

Such benefits make true-believer no-till practitioners steadfastly committed to keeping steel out of the ground. But sometimes there are good reasons for dusting off the tillage equipment. No till can cause low-mobility nutrients like phosphorous to accumulate near soil surface; without careful traffic management it can cause soil compaction; weeds can get away; or high-yield years can create excess crop residue that is difficult to plant into. The question is how much soil health, recovered after years of no till, is lost with occasional tillage?

Even a single tillage operation can increase soil aeration and accelerate loss of carbon and nitrogen, resulting in immediate bursts of carbon dioxide (CO2) and nitrous oxide (N2O), which are very sensitive indicators of soil disturbance and organic matter loss. Carbon dioxide and N2O are greenhouse gases that drive global warming, and agricultural practices that include frequent tillage are important contributors. Soils under no till tend to store more carbon and nitrogen so that losses as CO2 and N2O are reduced.

So, would a single tillage event in a long-term no-till field create huge bursts of CO2 and N2O when stored organic matter is exposed to air? To find out, we conducted an experiment at the University of Wyoming Sustainable Agriculture Research and Extension Center (SAREC) near Lingle.

We collected and compared soil air samples and surface soil samples from winter wheat-fallow systems that have been under no-till and frequent tillage management since UW bought the research station 11 years earlier. Small plots in each of the study fields were established to create three treatments: 1) no-till; 2) one-time till in no-till; and 3) summer tillage (typical) fallows. Air and soil samples were collected before and immediately after a one-time pass with a tandem disk that loosened the dry soil to 4-inch depth in the one-time till in no-till and the frequently tilled plots, and concurrently without tillage in the no-till plots. Carbon dioxide and N2O emissions, as well as concentrations of easily decomposed forms of organic matter were determined 0, 1, 5, 25, and 50 hours after tillage. To collect soil air, we drove a sharpened six-inch long piece of eight-inch diameter PVC pipe about three inches into the soil and left it there throughout the experiment. At each sampling time we placed a PVC cap fitted with a septum onto the base and sealed it with a rubber gasket. We used a syringe to collect soil air samples from the canister three times, at 0, 15, and 30 minutes, placing each sample in to a glass vacuum vial. The samples were analyzed by gas chromatography, and emissions calculated in micrograms per square meter of each gas.

Results indicate that CO2 emissions from one-time tillage in no-till were 30 to 40% lower than from the frequently tilled soils. The one-time tilled soils also had less soluble organic carbon and mineral nitrogen, indicating less disruption of organic matter. Surprisingly, the values from one-time tilled no-till soils were much closer to those from the no-till plots that we didn’t till than to the frequently tilled plots. This suggests that soil organic matter stored in the long-term no-till soil was resistant to the single summer tillage operation. Importantly, tillage did not affect the magnitude of N2O emissions in any of the treatments, suggesting that if performed during dry summer, this operation did not contribute to nitrogen loss as gas.

The results suggest that the 11 years of no till facilitated formation of stable soil aggregates that protected organic matter and were not destroyed by the tillage operation, and also formation of resistant organic compounds, like humus, that are not easily decomposed. Experience shows that repeated tillage would breakdown those materials, compromising the resilience gained during the years of no till. But occasional summer tillage to accomplish particular management objectives may not destroy the benefits of long-term no till, at least on the silt-loam soils at SAREC.

A more in-depth article about this research can be found here:

Greenhouse Gas Fluxes and Soil Carbon and Nitrogen Following Single Summer Tillage Event, International Journal of Plant and Soil Sciences

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