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SOIL FERTILITY & PLANT NUTRITION

An Improved Model of Nitrogen Release for Surface-applied Controlled-release Fertilizer

Dep. of Soil Science, Univ. of Wisconsin, 1525 Observatory Dr., Madison, WI 53706-1299

^* Corresponding author (rfujinuma{at}gmail.com).

Empirical temperature models are commonly used to estimate N release from polymer-coated controlled-release fertilizers (CRFs) under field conditions where the influence of soil moisture is negligible. For surface-applied CRF in bare-root nurseries, the effect of soil moisture may be important. To quantify the effect of soil moisture on N release from surface-applied CRF, two experiments were conducted in a sandy-textured soil: (i) a laboratory experiment to develop two semiempirical models that characterized the effect of soil temperature (T model) and moisture (MT model) on N release, and (ii) a field experiment to evaluate the precision of these models. In the laboratory experiment, the N release from CRF (urea, 41–0–0, 2-mo release) was measured under 16 combinations of four soil temperatures (5, 10, 23, and 35°C) and four soil moisture contents (0.002, 0.04, 0.11, 0.22 kg kg^–1). The calculated time constants for each abiotic combination showed that both soil temperature and soil moisture content controlled the cumulative N release (CNR). The difference between model estimates and field measurements of CNR indicated that the T model significantly underestimated CNR by as much as 18% (P < 0.01). In contrast, this difference was reduced to 8% using the MT model; the slope of the least-square fit for the MT model was not significantly different from unity at the 1% level and just significant at 5% (P = 0.052). Therefore, we argue that a model parameterized by both soil temperature and soil moisture will improve estimates of CNR from surface-applied CRF in irrigated tree nurseries.

Abbreviations: CNR, cumulative nitrogen release • CRF, controlled-release fertilizer