Root Parameters Show How Management Alters Resource Distribution and Soil Quality in Conventional and Low-Input Cropping Systems in Central Iowa

Abstract

Plant-soil relations may explain why low-external input (LEI) diversified cropping systems are more efficient than their conventional counterparts. This work sought to identify links between management practices, soil quality changes, and root responses in a long-term cropping systems experiment in Iowa where grain yields of 3-year and 4-year LEI rotations have matched or exceeded yield achieved by a 2-year maize (Zea mays L.) and soybean (Glycine max L.) rotation. The 2-year system was conventionally managed and chisel-ploughed, whereas the 3-year and 4-year systems received plant residues and animal manures and were periodically moldboard ploughed. We expected changes in soil quality to be driven by organic matter inputs, and root growth to reflect spatial and temporal fluctuations in soil quality resulting from those additions. We constructed a carbon budget and measured soil quality indicators (SQIs) and rooting characteristics using samples taken from two depths of all crop-phases of each rotation system on multiple dates. Stocks of particulate organic matter carbon (POM-C) and potentially mineralizable nitrogen (PMN) were greater and more evenly distributed in the LEI than conventional systems. Organic C inputs, which were 58% and 36% greater in the 3-year rotation than in the 4-year and 2-year rotations, respectively, did not account for differences in SQI abundance or distribution. Surprisingly, SQIs did not vary with crop-phase or date. All biochemical SQIs were more stratified (p<0.001) in the conventionally-managed soils. While POM-C and PMN in the top 10 cm were similar in all three systems, stocks in the 10–20 cm depth of the conventional system were less than half the size of those found in the LEI systems. This distribution was mirrored by maize root length density, which was also concentrated in the top 10 cm of the conventionally managed plots and evenly distributed between depths in the LEI systems. The plow-down of organic amendments and manures established meaningful differences in SQIs and extended the rhizosphere of the LEI systems. Resulting efficiencies observed in the LEI grain crops indicate that resource distribution as well as abundance is an important component of soil function that helps explain how LEI systems can maintain similar or greater yields with fewer inputs than achieved by their conventional counterparts.

Whole-profile soil organic matter content, composition, and stability under cropping systems that differ in belowground inputs

“…The vast majority of SOC (> 90 %) was associated with silt and clay particles at all soil depths, a finding that is consistent with results from other fractionation studies conducted on finely-textured Mollisols of the Midwest U.S. (Brown et al, 2014;Cates and Ruark, 2017;Lazicki et al, 2016). However, the distribution of MAOM between the microaggregate-associated silt plus clay (μSilt + Clay) and easily-dispersed silt plus clay (dSilt + Clay) shifted with depth, probably reflecting decreasing amounts of plant-and microbial-derived C and associated biological activity.…”

Can mineralization of soil organic nitrogen meet maize nitrogen demand?

“…The lack of a significant cropping system effect on gross ammonification in the present study is likely due in part to limited cropping system effects on SOM. While the diversified cropping systems did maintain modestly greater labile soil C (particulate organic matter C) compared to the 2-year system in the top 20 cm of soil (Lazicki et al 2016), similar differences could not be observed in the total SOM pool despite 12 years of consistent management differences between the cropping systems (Appendix), possibly because our experiment was established on Mollisols with high pre-existing SOM levels that may be C saturated (Brown et al 2014). Thus the modest differences in labile C may not have led to observable changes in gross ammonification.…”

Legacy effects of long-term nitrogen fertilizer application on the fate of nitrogen fertilizer inputs in continuous maize

“…Alternatively, improved soil physical properties (e.g., lower bulk density; Table 1) may have allowed roots to access water and nutrients more easily in plots with greater SOC content, increasing yield potential and NR crop (Unger and Kaspar, 1994). In another Iowa study, Lazicki et al (2016) reported higher yields and greater root length density of maize grown in cropping systems receiving more organic C inputs. Further research on root growth characteristics and root fertilizer N recovery across gradients of SOM may help to explain the curvilinear response NR crop observed in our study.…”