Kavdir, Y. 2000. Distribution of cover crop nitrogen retained by soil aggregates within a rye-corn agroecosystem. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

Decomposition of rye root and shoots and their contributions to soil N, its location within soil aggregates, and uptake by succeeding corn were monitored using two different field experiments in a Kalamazoo loam soil. Experiments were conducted at the LTER Interactions Sites and Microplots at the Kellogg Biological Station in Southwest Michigan from November 1997 to November 1999. Four main treatments were considered at the Interactions sites: Conventionally tilled with N fertilization (CT-F), conventionally tilled with no N fertilization (CT-NF), no tillage with N fertilization (NT-F) and no tillage with no N fertilization (NT-NF). Each plot was split in half with a rye cover crop planted on the west one half of the 16 plots. Sixteen microplots consisted of four treatments: bare fallow ©, bare fallow with rye shoot (RS), rye root (RR), and rye root plus shoot (RRS).

Rye plants were labeled with 15 N by foliar applications of solutions and 15 N enrichments of soil aggregates between 2.0-4.0, 4.0-6.3 and 6.3-9.5 mm across were determined after residue application. Concentric layers of aggregates were removed from each aggregate by meso soil aggregate erosion (SAE) chambers. Deep soil samples (to 150 cm) were collected to determine extractable inorganic N contents and to extract roots. Volumetric soil water contents were measured by TDR. Nondestructive development of roots was monitored by minirhizotron technology. Suction lysimeter samples were collected for soluble N content. Rye root uptake of residual soil N reduced inorganic N leaching from the soil profile. Dual spray applications of Roundup herbicide to Roundup-ready corn, resulted in the contributions of at least 28 kg N per ha (NT-NF) to 41 kg N per ha (NT-F) to the 1998 corn crop. Reducing nitrogen losses to groundwater beyond the root zone by 27 to 73 kg N per ha during the year. Negative linear correlations were observed between inorganic N contents and root length, volume and surface area in Ap horizons of all treatments. Rye roots deposited N to the surfaces of soil aggregates more rapidly than did rye shoots. Concentration gradients of recently derived rye N, within soil aggregates increased with aggregate size. Yields became more dependent on location of N within aggregates when soil aggregate size and N gradients increased. Recovery of rye shoot-derived 15 N by corn averaged 8% during this 2-year study. Soil aggregates from rye cover crop treatments were 2-10 fold more resistant to erosive forces applied by the SAE chambers than those from no rye cover crop treatments. In summary, short-term contributions of rye root and shoot N can be identified more rapidly when soil aggregates are peeled into different concentric layers and analyzed. Nitrogen flux rates and stability changes among the concentric layers and internal regions were reported within soil aggregates greater than 4 mm across. These gradients were identified in larger but not smaller soil aggregates and suggest alternative models are needed for predicting the formation of soil aggregates.

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