Huang, B. 1995. Tillage modifications of root and shoot growth responses to soil water content and nitrogen concentration altered by seasons. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

Conventional and no tillage modifications of soil water contents and their subsequent effects on crop growth are uncertain. This study was designed to quantify the soil water content under conventional tillage (CT) and no-tillage (NT) treatment systems associated with nitrogen patterns in solution through two growing seasons. Maize root and shoot responses to tillage-modified soil water contents were quantified in field experiments conducted on a Kalamazoo loam soil (fine loamy, mixed, mesic, Typic Hapludalf) at the Kellogg Biological Station in Michigan for 1991 and 1992. This replicated study consisted of conventional moldboard plus secondary tillage and no-tillage treatments which were fertilized with 143 kg N ha$\sp{-1}$.

Conventional tillage resulted in significantly lower soil water contents in 1991. Lower soil water contents limited plant uptake of nitrogen, the growth rate and accumulation of dry matter under CT conditions, resulting in significant reductions in grain yield. The grain yield of NT was 43% higher than that of the CT treatment in 1991. Significantly greater soil water contents for both tillage treatments in 1992 caused greater nitrate leaching, especially in the wetter NT treatments. Total recovery of nitrogen in plant shoot tissue and in the root zone portion of the soil profile was 88% in CT treatment and 59% in NT treatment during the period from 40-130 DAP, in 1992. Root growth in the no-tillage treatments was 73% and 100% greater than the CT treatments for both years. Soil water content dramatically influenced root dynamics, defined as the net accumulation of roots within the soil profiles. The greatest changes and contrasts in root growth and/or death rates occurred between CT and NT treatments during periods when soil water contents were low for both years.

Errors in predicted grain yields by CERES-Maize model were adjusted by reducing the precipitation which simulated surface runoff, reducing the amount of precipitation entering the root zone. Adjusted precipitation greatly improved the prediction of maize yields in 1991, suggesting that soil water content may have been the primary factor influencing yields in 1991.

Results of this study suggest that conventional tillage limits the grain yields of maize, especially during years when soil water content limited plant growth. Nitrate leaching from NT soils are greater during years when soil water contents were high. This tillage by soil water content interaction results in greater potentials for nitrate losses from soils receiving no tillage. Greater NO3 leaching from the root zone of wet NT soils creates a management dilemma which could be alleviated by additional studies designed to investigate plant root-mediated intervention of leaching from the root zone.

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