NITROGEN MANAGEMENT IN MULTI-SPECIES DAIRY PASTURES.

In a comprehensive study in the Tsitsikamma, two objectives were addressed: a) whether high N fertilization rates traditionally believed to increase pasture productivity, really achieve optimal yields and efficient N use, and b) how different nitrogen fertilizer rates influence soil nitrogen cycling processes, especially nitrogen mineralization from soil organic matter, and how these dynamics affect pasture productivity.

For objective a), an observational study was conducted using data from 153 commercial dairy pastures over five years. The pastures were classified according to their annual N fertilizer application rates into three categories: low (<200 kg N per ha), moderate (200 to 350 kg N per ha), and high (>350 kg N per ha). Statistical analyses, including mixed models and regression techniques, were used to evaluate the relationships between fertilizer rates, annual pasture yield, nitrogen use efficiency, and seasonal yield patterns.

Contrary to conventional expectations, the results showed that the pastures which received the highest N fertilizer rates (>350 kg N per ha), did not produce significantly higher annual herbage yields compared to fields receiving the moderate or lower applications. A weak but statistically significant negative correlation was observed between annual yield and N fertilizer rates, indicating that excessive N application may suppress pasture productivity. The N use efficiency was highest at the lower N fertilizer applications. The seasonal analyses revealed that optimal pasture yields occur in spring and summer, coinciding with natural N mineralization in the soil.

For objective b), soil samples were taken from the same 153 dairy pastures. The analysis included measurements of total N, nitrate N, ammonium N, potentially mineralizable N, total C, active C, and C to N ratios. Statistical methods, including correlation and regression analyses, were used to evaluate the effects of N fertilizer treatments on soil N mineralization and pasture yields.

The high N fertilization application (>350 kg N per ha) significantly reduced the soil's ability to mineralize organic N into forms available for plant uptake. The pastures which received these high N levels accumulated more nitrate in the soil, which negatively correlated with pasture yields. High nitrate levels indicate increased risks of nutrient leaching, groundwater contamination, and reduced pasture productivity. The pastures which received the low N inputs (<200 kg N per ha) maintained higher levels of ammonium N, which was positively correlated with improved pasture yields. These pastures demonstrated healthier N cycling and improved microbial activity.

In conclusion: The results showed that moderate fertilization (200 – 250 kg N per ha per year) will be more economical and environmentally sustainable than higher levels. Since optimal pasture growth occurs in spring and summer in this region, N fertilizer applications should ideally align with these periods of increased soil biological activity. The results, furthermore, suggest that strategically applying low fertilizer rates, coupled with an understanding of natural soil biological processes, can maintain or enhance pasture productivity, optimize N efficiency, and minimize environmental impacts.