For the past several decades, farmers have been abandoning their plows in favor of a practice known as no-till agriculture. Today, about one-third of U.S. farmers are no longer tilling their fields, and still more are practicing conservation tillage--using equipment that only disturbs the soil to a minimal degree. No-till and, to a lesser degree, conservation tillage maintains or improves soil quality by preserving soil structure and moisture, increasing soil organic matter, and providing habitat for soil microbes.
It's the microbes that matter most.
"Soil microbes are the workhorses of the soil. They break down crop residues and release nitrogen, phosphorus, potassium, and other nutrients back to the soil so they're plant-available. We want a healthy, diverse microbial community so that those processes can happen and improve our soils," says University of Illinois doctoral student Stacy Zuber.
Until now, most studies linking tillage intensity and microbial activity have been done at the scale of individual farms. Most of these studies do find more soil microbes with no-till management, but the magnitude of that result varies a lot from farm to farm. That's because each farm is influenced by different environmental factors, agronomic practices, and soil type. Where no-till is compared with tillage, the type of equipment and tillage depth also differs.
Zuber wanted to cut through the confusion to detect a true "signal" of the effect of tillage on soil microbes. To do that, she compiled and analyzed data from 62 studies from all across the globe.
"When you're doing individual field experiments--even if you have several in one area--you're still focused on the one region," Zuber notes. "Sometimes it's hard to see the big picture because there's so much variability. The meta-analysis allowed us to look at different field studies from around the globe to determine the overall effect. This process lets us see that big picture."
Zuber compared measures of microbial biomass and metabolic activity in no-till and tilled systems. For tilled systems, she included categories that accounted for the type of tillage equipment and tillage depth. She also accounted for the nitrogen fertilization rate, mean temperature and precipitation, the presence or absence of cover crops, and other variables.
When the data from all 62 studies were analyzed together, it turned out that microbial biomass and enzymatic activity were greater in no-till than in tilled systems. In tilled systems, the type of tillage equipment mattered. In contrast to other tillage equipment, such as moldboard plows or disc plows, the use of chisel plows was associated with greater microbial biomass. Chisel plows, which theoretically result in minimal soil disturbance, are commonly used as part of a conservation tillage system.
But experimental use of a chisel plow, as represented in the studies Zuber analyzed, may be different from how they are used in the real world.
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