Stratification of soil chemical and microbial properties under no-till after liming
Introduction
The fertile soils of the Palouse, a dryland cropping region of the inland Pacific Northwest (PNW) in the United States, have been under predominantly conventional tillage (CT) for nearly a century, meaning that soils were annually inverted to a depth of up to 0.3 m and thoroughly mixed. However, because much of the region is characterized by expansive, undulating hills of loess-derived soils, erosion has historically been significant, threatening the region’s agricultural sustainability. While CT remains the dominant form of cultivation practiced in the region, the acreage of agricultural land under no-till (NT) (lacking soil inversion) has grown and is greatly reducing soil erosion losses (McCool et al., 2001).
Growers in the inland PNW have been converting to NT for many reasons, including increased concentrations of soil organic carbon (Brown and Huggins, 2012), increased microbial biomass and activity (Alvear et al., 2005, Peigné et al., 2007), reduced greenhouse gas emissions (Stockle et al., 2012, Gibbon et al., 2014), and increased overall soil health (Triplett and Dick, 2008, Morrow et al., 2016). The lack of soil disturbance under NT may also promote unwanted consequences (Sullivan et al., 2016). Reduced mechanical mixing of the soil, combined with subsurface (4–8 cm depth) banding of ammonia-based fertilizers can lead to exacerbated soil nutrient stratification with extremely low pH, below 5.0 in many cases, reported at and below the fertilizer-placement zone (Brown et al., 2008, García-Marco et al., 2014, Karlen et al., 2013).
Soil acidification is becoming a significant concern in the Palouse, where the pH of agricultural surface soils (0–30 cm) have decreased from near-neutral pH in the 1960s (Mahler and Harder, 1984) to less than pH 5.2 in 21% of surveyed soils in 1995 (Mahler, 2002). The zone surrounding fertilizer placement is of particular interest in NT due to problems with exacerbated acidification. In one NT research field located in this region, the soil pH ranged from 5.1 in the top 0–5 cm to 6.3 in the lower 20–30 cm with a pH of 4.7 at the precise depth of fertilizer placement (Brown et al., 2008). This is particularly problematic because this pH is below the critical level for optimal yields of cereals (pH 5.2–5.4) or legumes (pH 5.4–5.6), which are the top two economically significant crops in the inland PNW (Mahler and McDole, 1987, Mahler and Harder, 1984).
Soil acidification poses a hurdle to agricultural productivity in many regions around the world, primarily due to the increased aluminum (Al) bioavailability that is associated with soil acidity. Aluminum-containing soil minerals undergo pH-dependent dissolution in acidic environments, which increases concentrations of bioavailable forms of Al in soil solution (Silva, 2012, Soti et al., 2015, Sposito, 2008). Dissolved Al3+ in soil solution interferes with crop growth by competing with plant nutrients, including calcium (Ca), manganese (Mn), phosphorous (P), and iron (Fe) (Seguel et al., 2015). This competition with plant nutrients results in deficiencies, which along with general Al phytotoxicity, can significantly reduce crop yields and soil productivity.
Soil acidification and impacts on crop yield in recent years have boosted interest in farm management practices that raise soil pH and/or slow the acidification process. Lime, or CaCO3, applications are often effective at increasing soil pH in addition to improving overall soil health (Bennett et al., 2014) by increasing soil total organic carbon concentrations and enhancing many aspects of soil structure (Mrabent et al., 2017). In soils of many tropical regions, lime applications have increased soil productivity and yields (Ila'ava et al., 2000, Laxminarayana et al., 2015, Nolla et al., 2013) and can be effective to great depths, even in NT systems (Caires et al., 2008, Costa et al., 2016). However, lime represents a substantial cost to farmers (Gibbons et al., 2014), and finding local sources of lime may significantly reduce overall costs while also resulting in a variety of lime materials with distinct characteristics and impacts (Carrizo et al., 2014).
Both soil pH and Al bioavailability, as well as subsequent liming, can influence microbial community structure and function. While NT can intensify soil stratification of pH and Al bioavailability, little is known regarding the response of the microbial communities to zones of acidification in soils of the inland PNW. Even less is known concerning the dynamic interactions of the microbial community with locally-sourced liming materials on stratified soil pH and Al bioavailability (Brown et al., 2008). Additionally, detailed characterization of NT stratified soil at highly resolved depth increments (2-cm) has not been previously used to determine the effectiveness of liming in the fine-silty soils of the arid, dryland cropping region of the inland PNW. Therefore, the objectives of this study were to: (1) determine whether soil sampling at small, discrete depth increments captures stratification of chemical and microbial properties in NT soils, and (2) evaluate the soil chemical and microbial impacts of two types of locally-sourced, surface-broadcasted lime materials at each depth interval.
Section snippets
Field sites and liming treatments
This study was initiated in the fall of 2013 at two sites within the Palouse region of the inland PNW with an established history of continuous NT. The first site is located on the Palouse Conservation Field Station (PCFS) near Pullman, WA (46° 45′N, 117° 11′W, 800 m a.s.l.) and is thoroughly described by Brown et al. (2008). Soil at the PCFS site formed under native prairie vegetation and the dominant soil series is Thatuna silt loam (Fine-silty, mixed, superactive, mesic, Oxyaquic Argixeroll)
Soil chemistry
Patterns along the highly resolved (2-cm) depth increments in the measured soil chemical variables did not result in a significant site-by-treatment interaction, which allowed results from both PCFS and RF to be displayed together, providing greater statistical power to evaluate effects of depth, as well as interactions of depth, time, and treatment (Table 2, Table 3).
Soil pH increased in both treatments and in all depths compared with the control at 6 mo (Table 2). Without lime applications,
Discussion
Typical sampling depths for characterization of soil chemistry and microbial communities range from 0 to 15 cm (Carpenter-Boggs et al., 2003, Rütting et al., 2013, Su et al., 2015) or 0–20 cm (Wang et al., 2013, Zhi-An et al., 2008), and 0–30 cm (Yvanes-Giuliani et al., 2014), which may be useful to growers or for regional comparison, but grossly overlooks the dramatic, micro-scale impacts of no-till stratification. In our study, when depth was removed from the statistical model and each
Conclusions
The 2-cm spatial resolution employed for soil analyses in our study provided insight on soil chemical stratification and microbial community composition, revealing differences between the top and bottom depths that would not have been realized in a 10 cm core. The spatial resolution allowed observation of the precise depth of treatment effects on the soil chemistry and microbial communities, and movement of treatment effects, as well as Al and other metals, over time. It also clarified the
Acknowledgements
The authors would like to gratefully acknowledge E. Torres and J. DeAvila for technical assistance throughout the duration of the project and M. Mazzola for assistance with molecular analysis.
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