Effects of Short-term Tillage of a Long-term No-Till Land on Quantity and Quality of Organic C and N in Two Contrasting Soil Types

Pre-seeding tillage of long-term no-till soil may alter soil quality by changing some properties, but the magnitude of change depends on soil type and climatic conditions. Effects of short-term (2 or 3 years) tillage (hereafter called reverse tillage [RT]) of land previously under long-term no-till (NT, 29 or 30 years), with straw management (straw removed [SRem] and straw retained [SRet]) and N fertilizer rate (0, 50 and 100 kg N ha-1 in SRet, and 0 kg N ha-1 in SRem plots) were determined in autumn 2011 on total organic C (TOC) and N (TON), light fraction organic C (LFOC) and N (LFON), and mineralizable N (Nmin) in the 0-7.5, 7.5-15, or 15-20 cm soil layers at Breton (Gray Luvisol [Typic Cryoboralf] loam) and Ellerslie (Black Chernozem [Albic Argicryoll] loam), Alberta, Canada. Short-term RT following long-term NT had no significant negative effect on TOC and TON in soil at both sites, although these parameters tended to be slightly lower in the 0-7.5 cm soil layer with RT compared to NT. For the zero-N treatment, SRet had greater TOC and TON compared to SRem in both soil layers at both sites. On average, over both sites, TOC and TON in the 0-15 cm soil increased by 2.08 Mg C ha-1 and 0.216 Mg N ha-1, respectively. Application of N fertilizer increased TOC and TON in both soil layers, up to the 50 kg N ha-1 rate at Breton (by 7.96 Mg C ha-1 and 0.702 Mg N ha-1 in the 0-15 cm soil) and up to the 100 kg N ha-1 rate at Ellerslie (by 5.11 Mg C ha-1 and 0.439 Mg N ha-1 in the 0-15 cm soil). In both RT and NT treatments, the effects of N rate on TOC and TON were similar for SRet and SRem. There was greater LFOC and LFON in the 7.5-15 cm soil layer with RT than NT at both sites. In the 0-15 cm soil layer, averaged over both sites, RT increased LFOC by 66 kg C ha-1 and LFON by 4.0 kg N ha-1. In both 0-7.5 and 7.5-15 cm soil layers, LFOC and LFON increased with SRet compared to SRem. Averaged over both sites, the increase in LFOC and LFON in the 0-15 cm soil was 97 kg C ha-1 and 3.5 kg N ha-1, respectively. Mass of LFOC and LFON increased dramatically in both soil layers with application of N fertilizer up to the 100 kg N ha-1 rate at both sites, with an average increase of 866 kg C ha-1 and 45.5 kg N ha-1. In the zero-N treatment, LFOC and LFON increased with SRet compared to SRem under RT at Breton and under NT at Ellerslie. On average, tillage had no effect on Nmin in soil, but SRet increased Nmin in soil in both RT and NT, with an average increase of 4.8 kg N ha-1. Application of N fertilizer increased Nmin in the 0-20 cm soil up to 50 kg N ha-1 rate at Breton (by 13.7 kg N ha-1) and up to 100 kg N ha-1 rate at Ellerslie (by 18.6 kg N ha-1). In conclusion, RT had no effect on TOC, TON and Nmin in soil, but LFOC and LFON increased with RT compared to NT in the 7.5-15 cm layer at one site. SRet and N fertilization usually had dramatic positive effects on TOC, TON, LFOC, LFON and Nmin in soil compared to the corresponding treatments.