Evaluating novel biodegradable polymer matrix fertilizers for nitrogen‐efficient agriculture

Enhanced efficiency fertilizers (EEFs) can reduce nitrogen (N) losses in temperate agriculture but are less effective in the tropics. We aimed to design a new EEF and evaluate their performance in simple-to-complex tests with tropical soils and crops. We melt-extruded urea at different loadings into biodegradable polymer matrix composites using biodegradable polyhydroxyalkanoate (PHA) or polybutylene adipate-co-terephthalate (PBAT) polymers with urea distributed throughout the pellet. These contrast with commercially coated EEF that have a polymer-coated urea core. We hypothesized that matrix fertilizers would have an intermediate N release rate compared to fast release from urea or slow release from coated EEF. Nitrogen release rates in water and sand-soil columns confirmed that the matrix fertilizer formulations had a more progressive N release than a coated EEF. A more complex picture emerged from testing sorghum [Sorghum bicolor (L.) Moench] grown to maturity in large soil pots, as the different formulations resulted in minor differences in plant N accumulation and grain production. This confirms the need to consider soil interactions, microbial processes, crop physiology, and phenology for evaluating fertilizer performance. Promisingly, crop δ15N signatures emerged as an integrated measure of efficacy, tracking likely N conversions and losses. The three complementary evaluations combine the advantages of standardized high-throughput screening and more resource-intensive and realistic testing in a plant-soil system. We conclude that melt-blended biodegradable polymer matrix fertilizers show promise as EEF because they can be designed toward more abiotically or more microbially driven N release by selecting biopolymer type and N loading rate.