Long‐Term Crop Residue and Nitrogen Management Effects on Soil Profile Carbon and Nitrogen in Wheat–Fallow Systems

Abstract

Intensive cultivation of native grassland for dryland agriculture continuously depleted soil organic carbon (SOC) and nutrients. In 2010, we evaluated the infl uence of 80 yr of crop residue and nutrient management practices on SOC and N in 0-to 60-cm soil depth profi les in conventionally tilled winter wheat (Triticum aestivum L.)-summer fallow (WW-SF) system. Residue and N treatments, no N addition with fall burning (FB0), spring burning (SB0), and no burning (NB0), 45 kg N ha -1 with SB (SB45) and NB (NB45), 90 kg N ha -1 with SB (SB90) and NB (NB90), manure (MN, 5.32 Mg dry mass ha -1 yr -1 ), and pea vines (PV, 0.99 Mg dry mass ha -1 yr -1 ), were in ordered arrangement, and an undisturbed grassland (GP) was used as a reference. All WW-SF treatments had less SOC and N stocks than GP. Th e SOC stocks were lowest under FB0 with 50% less SOC than GP. Th e WW-SF treatments have depleted up to 63 and 26% of SOC and N from surface soil since 1931. Fall burning and MN treatments depleted SOC at rates of 0.64 and 0.17 Mg ha -1 yr -1 . Nitrogen stocks decreased at a rate of 0.02 Mg ha -1 yr -1 in FB, SB, and NB treatments, and 0.01 Mg ha -1 yr -1 in PV treatment. Reduction in tillage, application of low C/N ratio residues, and elimination of burning can improve sustainability of winter wheat production in the summer fallow region of the Pacifi c Northwest (PNW).

Soil Organic Carbon Pools as Early Indicators for Soil Organic Matter Stock Changes under Different Tillage Practices in Inland Pacific Northwest

“…Approximately 10 g subsamples of air-dried soils (<2 mm) were finely ground (<0.05 mm) in a Shatter Box 8530 ball mill (Spex Sample Prep., Metuchen, New Jersey, USA) for 3 min, and then analyzed for total C and N concentrations by dry combustion method (Purakayastha et al, 2008) at 950 • C using a LECO CN628 analyzer (LECO Corp., St. Joseph, Michigan, USA). Previous investigations have confirmed the absence of inorganic carbon at 0-15 cm layer within the experimental site, and therefore, total C measured for all soils is safely assumed to be soil organic carbon (SOC) (Ghimire et al, 2015). This was confirmed by pH-values measuring below 6.7 in all soils ( Table 1).…”

“…For the current study site, the mean aboveground plant biomass inputs from GP and wheat-pea systems were estimated to be about 7 and 5 mg ha −1 year −1 , respectively (Machado, 2011;Ghimire et al, 2015). Researchers have also asserted that undisturbed grasslands usually contain greater root density than cultivated systems (Gregorich et al, 2000;Beniston et al, 2014).…”

“…Soil bulk density was used to convert SOC concentration (g kg −1 ) to SOC stock per area (Mg ha −1 ) to remove confounding effects of compaction when comparing all treatments. Soil pH of extracts from 5 g air-dried soils (<0.05 mm) in 10 mL of 0.01 M CaCl 2 was measured electrometrically using an Orion Star A215 pH/conductivity bench top meter (Thermo Fisher Scientific Inc., Beverly, MA, USA) (Ghimire et al, 2015). Water holding capacity (WHC) of air-dried soil (<2 mm) was determined as described by Awale and Chatterjee (2015).…”

“…In addition, tillage also facilitates seeding operations by removing surface residues and reducing weed germination by burying weed seeds (Young et al, 2014). Although reliable in terms of grain yield, WW-SF exacerbated soil erosion (Feng et al, 2011;Machado et al, 2015) and has depleted more than 50% of the original SOM in Walla Walla silt loam soil (0-60 cm) near Pendleton, Oregon (Ghimire et al, 2015). In response to high soil erosion rates and depletion of SOM, adoption of reduced tillage systems, including delaying tillage until crop seeding, minimum tillage, and no-till (NT), has increased in recent years (Machado, 2011;Machado et al, 2015).…”

“…More importantly, due to the slow recovery of SOM stocks, it may take several years to observe significant changes in SOM, often leading to late decision making and delayed remedial actions (West and Post, 2002). It took more than 30 years to measure a significant decrease in SOM in a WW-SF long-term experiment near Pendleton, Oregon (Ghimire et al, 2015). Besides, other studies have also indicated that the magnitude and direction of tillage-induced changes are often site-specific (Purakayastha et al, 2008;Morrow et al, 2016).…”