Fertilizer-nitrogen and cover-crop rotation effects on inbred maize (Zea mays L.) yield and soil quality
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This thesis evaluates the effects of fertilizer-nitrogen (N) and cowpea (Vigna unguiculata (L.) Walp.) cover crop (CC) rotation on inbred maize production, microbial community structure and activity and soil quality. The study was conducted in field plots established in the municipality of Guayama, Puerto Rico. The cropping systems evaluated were a maize-cowpea cover crop rotation vs a typical practice of maize fallow (FA), each with five fertilizer-N levels. The maize planting sequence was an initial spring 2014 planting, followed by a winter 2014-2015 planting and a final winter 2015-2016 planting. The cowpea cover crop was planted in the summers of 2014, 2015 and 2016. The inbred maize lines were SSH65VH in 2013-2014 and 2014-2015 and SLM15VH in 2015-2016. Mean maize plant densities ranged from 59,391 to 69,182 plants/ha for the cropping seasons. The soil plant analysis development (SPAD-502) chlorophyll meter® (leaf greenness), NDVI (normalized difference vegetation index measured with GreenSeeker®) and plant height measurements were used as indicators of crop N status. Fertilizer-N applications and CC rotation significantly influenced (p<0.05) leaf greenness and plant height. Fertilizer-N rates significantly influenced maize seed yield. Nutrient use efficiency (NUE) was measured as partial factor productivity (PFP), agronomic efficiency (AE), partial nutrient balance (PNB), apparent recovery utilization (RE), internal utilization efficiency (IE) and physiological efficiency (PE). Optimal NUE values were reached for PFP, AE and RE at 90 kg N/ha (2013-2015) and 50 kg N/ha (2015-2016). Non-linear regression models indicated fertilizer-N requirements of 143 kg N/ha at a seed yield of 6,917 kg/ha for the 2013-2014 season, 156 kg N/ha with a 4,579 kg/ha seed yield for the 2014-2015 season and 36 kg N/ha at a yield of 2,535 kg/ha for the 2015-2016 season. Fertilizer-N applications decreased soil pH with increasing N rates. The soil management assessment framework (SMAF) soil quality index (SQI) score was higher for cover crop treatment than fallow. The lowest mean SQI (%) score was obtained for the biological and biochemical category with 43. This indicates that more management practices that improve soil biology can be implemented. Fatty acid methyl esters (FAME’s) were used to assess microbial community composition and soil enzyme activity was used to assess microbial community activity. The microbial community was influenced by CC rotation but not by fertilizer-N applications. Bacterial relative abundance for CC rotation was 15.6% greater than FA rotation. Enzyme activity was also influenced solely by CC rotation. Results from a combined enzyme activity assay (β-glucosidase, β-glucosaminidase and acid phosphatase) showed higher total enzyme abundance in CC treatment when compared to FA. Overall fertilizer-N rates affected plant height, SPAD-502, seed yield, immediately available N (0-30 cm), potentially leached N (30-90 cm) and soil pH at varying seasons. Cover crop rotation during different seasons affected all three agronomic indicators (SPAD-502, NDVI and plant height), soil pH, potentially mineralizable N, immediately available N, and microbial community structure and activity.