Greater soil carbon stocks and faster turnover rates with increasing agricultural productivity

Abstract. More diverse crop rotations have been promoted for their potential to remediate the range of ecosystem services compromised by biologically simplified grain-based agroecosystems, including increasing soil organic carbon (SOC). We hypothesized that functional diversity offers a more predictive means of characterizing the impact of crop rotations on SOC concentrations than species diversity per se. Furthermore, we hypothesized that functional diversity can either increase or decrease SOC depending on its associated carbon (C) input to soil. We compiled a database of 27 cropping system sites and 169 cropping systems, recorded the species and functional diversity of crop rotations, SOC concentrations (g C kg/soil), nitrogen (N) fertilizer applications (kg NÁha ). We categorized crop rotations into three broad categories: grain-only rotations, grain rotations with cover crops, and grain rotations with perennial crops. We divided the grain-only rotations into two sub-categories: cereal-only rotations and those that included both cereals and a legume grain. We compared changes in SOC and C input using mean effect sizes and 95% bootstrapped confidence intervals. Cover cropped and perennial cropped rotations, relative to grain-only rotations, increased C input by 42% and 23% and SOC concentrations by 6.3% and 12.5%, respectively. Within grain-only rotations, cereal + legume grain rotations decreased total C input (À16%), root C input (À12%), and SOC (À5.3%) relative to cereal-only rotations. We found no effect of species diversity on SOC within grain-only rotations. N fertilizer rates mediated the effect of functional diversity on SOC within grain-only crop rotations: at low N fertilizer rates (≤75 kg NÁha ), the decrease in SOC with cereal + legume grain rotations was less than at high N fertilizer rates. Our results show that increasing the functional diversity of crop rotations is more likely to increase SOC concentrations if it is accompanied by an increase in C input. Functionally diverse perennial and cover cropped rotations increased both C input and SOC concentrations, potentially by exploiting niches in time that would otherwise be unproductive, that is, increasing the "perenniality" of crop rotations.

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“…, Sanderman et al. ). In some cases, however, lower total C input in one cropping system corresponds to higher SOC (Gregorich et al.…”

Section: Introductionmentioning

confidence: 99%

Crop rotations for increased soil carbon: perenniality as a guiding principle

King

Blesh

2017

Ecological Applications

157

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“…Hydrolysis with strong acid has been shown to isolate SOM that is consistently hundreds to thousands of years older than the bulk SOM (Paul et al, 2001). Biological fractionation involves the modeling of mass loss or CO 2 evolution during a laboratory incubation experiment (Grandy and Robertson, 2007;Schädel et al, 2013). Baldock et al (2013), recognizing that fire-derived pryogenic carbon (PyC) has distinct properties from plant or microbial products, combined physical size fractionation with the use of solid-state 13 C nuclear magnetic resonance spectroscopy to mathematically isolate the PyC fraction from each size fraction.…”

Section: Introductionmentioning

confidence: 99%

Ramped thermal analysis for isolating biologically meaningful soil organic matter fractions with distinct residence times

Sanderman

Grandy

2020

SOIL

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Abstract. In this work, we assess whether or not ramped thermal oxidation coupled with determination of the radiocarbon content of the evolved CO2 can be used to isolate distinct thermal fractions of soil organic matter (SOM) along with direct information on the turnover rate of each thermal fraction. Using a 30-year time series of soil samples from a well-characterized agronomic trial, we found that the incorporation of the bomb spike in atmospheric 14CO2 into thermal fractions of increasing resistance to thermal decomposition could be successfully modeled. With increasing temperature, which is proportional to activation energy, the mean residence time of the thermal fractions increased from 10 to 400 years. Importantly, the first four of five thermal fractions appeared to be a mixture of fast- and increasingly slower-cycling SOM. To further understand the composition of different thermal fractions, stepped pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS) experiments were performed at five temperatures ranging from 330 to 735 ∘C. The Py-GC/MS data showed a reproducible shift in the chemistry of pyrolysis products across the temperature gradient trending from polysaccharides and lipids at low temperature to lignin- and microbe-derived compounds at middle temperatures to aromatic and unknown compounds at the highest temperatures. Integrating the 14C and Py-GC/MS data suggests the organic compounds, with the exception of aromatic moieties likely derived from wildfire, with centennial residence times are not more complex but may be protected from pyrolysis, and likely also from biological mineralization, by interactions with mineral surfaces.

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“…Typical of the results obtained using this approach, Sanderman et al . ( 4 ) found that bulk SOC turnover rates varied from 30 to 40 years in low-productivity crop-fallow system to under 10 years under a highly productive pasture for the same soil in a Mediterranean climate.…”

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confidence: 99%

“…But multipool modeling and chemical fraction methods reveal that there is also a highly resistant fraction of SOC that can be up to a few thousand years old ( 6 ). This highly resistant fraction can comprise 20 to 60% of the SOC ( 4 , 6 ), suggesting that this carbon was incorporated into the soil under a different climate than found today but the relevant paleoclimate period should be at most 2000 years before present.…”

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confidence: 99%