Effects of continuous negative pressure water supply on maize nitrogen uptake, leaf nitrate reductase activity and rhizosphere nitrogen supply

Objectives We investigated the effects of continuous negative pressure water supply on maize N uptake, leaves nitrate reductase activity (NR), alkali-hydrolyzed nitrogen content and urease activity in rhizosphere soil, so as to provide a theoretical basis for improving the efficiency of negative pressure water supply.

Methods Negative pressure watering experiments were carried out in the Experimental Base of Heilongjiang Bayi Agricultural University in 2019 and 2020, the maize variety Xianyu 335 was used as the test material. Taking conventional watering as control (CK), three negative pressure watering treatments (−5 kPa, −10 kPa and −15 kPa), were setup. At the main growing stages of maize, plant samples were collected for the analysis of leaf nitrate nitrogen content and nitrate reductase activity, and the nitrogen uptake in maize root, stem, leaf, bract, cob and grains, and the maize biomass and yield were measured; soil samples were collected at the same stage for the determination of water and alkali hydrolyzed nitrogen (AN) content, and the urease activities in rhizosphere soil.

Results The soil moisture content was not significantly different between CK and −5 kPa treatment during various growth stages of maize, while those in −10 kPa and −15 kPa treatments were significantly lower than CK; the maize root system, aboveground biomass and grain yield under −5 kPa treatment were the highest at maturity, which were 72.67%, 24.81% and 38.74% higher than CK in 2019, respectively, and the aboveground biomass and grain yield were 13.30% and 16.21% higher than CK in 2020, respectively; However, the tested indexes under −10 kPa and −15 kPa treatments were significantly lower than in CK. −5 kPa treatment promoted maize N uptake and utilization, the average N uptake of roots, stems, leaves, cobs, and grains was 24.92%, 24.86%, 32.01%, 18.07%, and 16.77% higher than CK in 2019, and 8.27%, 85.01%, 30.13%, 18.07%, and 16.77% higher than CK in 2020, respectively; while −10 kPa and −15 kPa treatments reduced the total N uptake of maize. The NO₃^--N content and NR activity in maize leaves under −5kPa treatment was significantly higher than under CK, but the those under −10 kPa and −15 kPa treatments were significantly reduced at tasseling and ripening stages of maize. The rhizosphere soil available N under −5kPa treatment was not significantly different from that of CK at jointing and large trumpet stage, but was 10.08% and 20.91% lower in 2019, and 25.88% and 31.93% lower than CK in 2020, respectively (P<0.05); while the rhizosphere soil available N under −10kPa, especially under −15kPa treatment were significantly higher than the control. The rhizosphere soil urease activity under −5 kPa treatment was significantly lower than that under CK throughout entire maize growth period, the soil urease activity were 4.12%, 20.44%, 23.59%, and 15.53% lower at jointing, big trumpet, tasseling and maturing stages in 2019, and 23.12%, 32.01% and 27.99% lower than CK at big trumpet, tasseling and maturing stages in 2020; but theurea activities under −10 kPa and −15 kPa treatment were higher all the time or equal to that under CK.

Conclusions Continuous water supply at negative pressure of −5 kPa can maintain soil moisture similar to conventional water supply level throughout the whole growth period, thereby promote N uptake and utilization, at the same time reduce rhizosphere soil available N content and urease activity. However, watering pressure −10 kPa and −15 kPa cannot meet the water supply of maize, so is not conducive to the growth and nitrogen uptake of maize.