Objectives The addition of additives, composed of modified montmorillonite and organic polymer adhesives, in urea has been proved of slow-releasing property in addition ratio of > 5%. However, according to a recently released National Standard of China, functional materials in urea products should be ≤ 2.5%. To date, there is little consensus about the performances of slow-release urea (SRU) with matrix materials at such low proportions. We developed a novel SRU with matrix materials at 2.5%, and studied its slow release property and effects on crop growth and yield attributes.

Methods A two-year field experiment was conducted during 2016 and 2017, following a randomized block design with three replicates. The experiment included three treatments, i.e., the control test (CK, without N application), common urea (CU), and SRU. Nitrogen application rate was N 150 kg/hm² for rice and 195 kg/hm² for maize. Shoot height, leaf area, root area, leaf chlorophyll concentration, leaf nitrate reductase activity, leaf glutamine synthetase activity, biomass, grain yield, yield components, fertilizer N leaching, fertilizer ammonia emission, and soil inorganic N concentration were assessed and compared among the treatments. Nitrogen loss via N leaching and ammonia emission was assessed in laboratory experiments.

Results The means of the two-year data were compared. Compared with common urea, SRU treatment tended to increase plant height (by 4.5% for rice and 12.4% for maize), leaf area (by 9.8% for rice and 11.5% for maize), root area (by 9.5% for rice and 5.0% for maize), leaf chlorophyll concentration (by 18.5% for rice and 7.8% for maize), leaf nitrate reductase activity (by 19.6% for rice and 20.3% for maize), and leaf glutamine synthetase activity (by 17.7% for rice and 11.5% for maize). Compared with CU treatment, SRU treatment significantly increased biomass of rice and maize in both growing seasons (P < 0.05). The yield increase of rice under SRU treatment was greater than that of maize. Rice grain yield in SRU treatment was 17.2% (P < 0.05) greater than that in CU treatment; while maize grain yield in SRU treatment was 6.6% (P > 0.05) greater than that in CU treatment. Compared with CU treatment, SRU treatment tended to increase panicle density and decrease grain number per panicle (for rice); while it tended to increase grain number per spike and thousand grain weight (for maize). Path analysis showed that the increases of rice grain yield in SRU treatment were mainly due to the increases of panicle density; while the increases of maize grain yield in SRU treatment were largely due to the increases of grain number per spike. Characteristics of fertilizer N loss (via N leaching and ammonia emission) were well described by logistic equations (P < 0.01). Parameters from the fitted equations showed that SRU had lower fertilizer N loss risks than CU. At anthesis stage, SRU treatment increased soil inorganic N concentration (in paddy fields) by 9.2% (P < 0.05); while it increased soil inorganic N concentration (in maize fields) by 18.1% (P < 0.05). The SRU treatment had lower risks of N leaching and ammonia emission, which were partly responsible for the greater soil inorganic N concentration and the improved crop performances in SRU treatment.

Conclusions The current slow release urea has good performance in reducing N loss, increasing soil inorganic N concentration, and improving rice and maize growth. Thus, it is a promising slow-release fertilizer for production of rice and maize.