Changes in Soil Organic Carbon and Nitrogen Fractions with Duration of No‐Tillage Management

Abstract

The influence of no-tillage (NT) management on labile soil N is poorly defined. To quantify changes in soil C and N fractions with duration of NT, we sampled Kempsville sandy loam (fine-loamy, siliceous, subactive, thermic Typic Hapludults) soils in the Coastal Plain of Virginia from farm fields that had similar cropping histories and nutrient management but varied in amount of time in continuous NT from 0 to 11 yr. At the 0- to 2.5-cm soil depth, there was a linear increase with time in NT for total organic C, total organic N, particulate organic matter (POM)-C, POM-N, POM-C/total organic C, POM-N/total organic N, hydrolyzable unidentified-nitrogen (hUN-N) and NH4–N (hNH4–N), and various biochemical classes of pyrolysate compounds determined by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). At the 2.5- to 7.5-cm soil depth, there was a linear increase with time in NT for total organic C and N, hUN-N, hNH4–N, hydrolyzable α amino acid-nitrogen (AA-N) and amino sugar-nitrogen (AS-N) only; these rates of increase were lower than at the 0- to 2.5-cm depth. No measured C and N fractions increased at the 7.5- to 15-cm soil depth but POM-C/total organic C and POM-N/total organic N declined (p ≤ 0.10) at this depth, suggesting that the recalcitrance of C and N fractions increased at 7.5- to 15-cm in 11 yr. These patterns indicate that a new equilibrium level for these C and N fractions had not been reached but that their stratification of C and N pools with soil depth increased during 11 yr in NT. At the 0- to 2.5-cm depth, the labile N fractions, AA-N and AS-N approached or reached new equilibrium levels within 11 yr. Since these fractions are derived from soil microbial and plant biomass, the initial rapid increase in these fractions likely reflects an increase in N immobilization in the short term (3–6 yr) following adoption of NT. After this initial transition period, levels of AA-N and AS-N approach new elevated equilibrium levels, which would be reflected in a higher net N mineralization rate than under the previous tilled system. These results indicate that absolute values for labile soil N fractions such as AA-N and AS-N may not be readily interpretable with respect to soil fertility recommendations without considering whether these values are changing over time or are at an equilibrium level. Different patterns of change in POM-N vs. AA-N and AS-N indicate that these labile N fractions are distinct and may provide different types of information with respect to quantifying changes in soil fertility with time under NT.