Soil Organic Carbon Pools As Early Indicators for Soil Organic Matter Stock Changes Under Different Tillage Practices in Inland Pacific Northwest.

The objectives of this study were to evaluate long-term effects of tillage intensity and timing on soil organic matter (SOM) pools and determine the most responsive SOM pools to tillage practice. Soil from the long-term (53 yrs) winter wheat (Triticum aestivum L.) - spring pea (Pisum sativum L.) rotation and undisturbed grass pasture (GP) in inland Pacific Northwest (PNW) was sampled to evaluate the effect of four tillage systems [no-till (NT), disk/chisel (DT/CT), spring plow (SP), and fall plow (FP)] on soil organic carbon (SOC, proxy for SOM), total nitrogen (TN), particulate organic matter carbon (POM-C) and nitrogen (POM-N), permanganate oxidizable carbon (POXC), water extractable organic carbon (WEOC) and total dissolved nitrogen (TDN), KCl-extractable nitrogen (KEN), microbial biomass carbon (MBC) and nitrogen (MBN), basal respiration (BR), carbon mineralization (Cmin), and metabolic quotient (qCO₂). The GP had higher levels of SOC pools than cultivated treatments. On average, tillage significantly decreased SOC and TN by 28% and 26%, respectively, compared to GP. Among the cultivated soils, tillage had no significant effect on SOC and TN, except for DT/CT that had slightly higher SOC than FP (P=0.08). On the contrary, NT and DT/CT significantly (P<0.05) increased the levels of POM-C, POM-N, POXC, WEOC, MBC, BR, Cmin, and qCO₂ over FP or SP. However, tillage did not affect TDN, MBN, and KEN. The C-pools (POM-C, POXC, MBC, WEOC, BR, and Cmin) were more strongly correlated with SOM than the N-pools (TDN, MBN, and KEN), with an exception to POM-N. Under wheat-pea rotation in the inland PNW, low disturbance tillage systems (NT and DT/CT) have a potential to maintain or increase SOM, which can be assessed early through its physical (POM), chemical (POXC, WEOC), and microbiological (MBC, BR, Cmin) indicators. POXC and WEOC were the most sensitive indicators of tillage induced changes in SOM dynamics.