Effects of straw mulching on growth and nutrients accumulation of direct-sown winter oilseed rape

Objectives The yield of winter oilseed rape is limited by seasonal drought, low temperature in winter and low soil fertility. Based on the advantages of straw returning to soil and the quantity of the surplus rice straw in the middle and lower reaches of the Yangtze River, field experiment was conducted to study the effect of straw mulching during the growing season of direct-sown winter oilseed rape in the rice-rapeseed rotation area. Changes of plant density, plant height and rhizome thickness, oilseed rape yield and nutrients uptake under straw mulching were explored.

Methods Two years (2014–2015 and 2015–2016) of field experiment were conducted at the test field of Huazhong Agricultural University in Wuhan, Hubei province. Four treatments were as followed: 1) Control (CK), without fertilization and no straw returning to field; 2) straw mulching to the field but no fertilization(S); 3) chemical fertilization only (NPK); 4) straw mulching and chemical fertilization (NPK + S). Fertilization treatments (NPK, NPK + S) recieved: N 180 kg/hm², P₂O₅ 60 kg/hm², K₂O 75 kg/hm² and borax 15 kg/hm². The samples were taken at seedling, bud-bolt, flowering, pod and maturity stages respectively to determine biomass, nitrogen, phosphorus, potassium contents and accumulation amount of oilseed rape. Oilseed rape density, plant height and rhizome thickness were measured in the field.

Results The straw mulching could increase the soil minimum temperature by 0.6–1.2 degrees (95 days after sowing), and reduce the highest soil temperature by 0.8–1.8 degrees centigrade (184 days after sowing), reduce soil temperature variation by 2.3 degrees centigrade (95–184 days after sowing) and increase soil average moisture content by 8.0%–8.9% (48–184 days after sowing). Straw mulching reduced the seedling density of winter oilseed rape by more than 80% compared with no mulching. The density of CK, S, NPK and NPK + S at maturity stage decreased by 71.3%, 40.3%, 69.5% and 32.1% respectively, compared with those at emergence. The density reduction range of oilseed rape with straw mulching was less than that of no mulching during the growth period. The rhizome thickness and plant height of the S treatment were 22.7% and 8.3% higher than those of CK treatment at mature stage, respectively. No significant difference was showed between NPK + S and NPK treatments. Two years’ field experiments showed that the maximum biomass of S treatment was increased by 88.6% on average compared with CK. Compared to the NPK treatment, the biomass of NPK + S treatment at seedling stage decreased by 3.7%–27.9%, but that in the pod stage increased by 28.1%. The CK and S treatments had significant differences in N, P and K accumulation at the bud-bolting and flowering stages. The accumulation of N, P of S treatment was higher than that of CK at maturity by 28.6%–268.2% and 93.3%–253.1% respectively, with the yield increment by 218.8% and 28.5% respectively. The accumulation of N, P and K in winter oilseed rape increased with the advance of growth season and reached the maximum at the pod stage with fertilization (NPK, NPK + S) treatments. The NPK + S treatment yielded N, P and K accumulation of 18.1%–19.1%, 23.7%–36.9% and 28.3%–56.9% higher than the NPK treatment at maturity stage, respectively. Compared with NPK treatment, oilseed rape yield of NPK + S increased by 1811 and 1032 kg/hm² during the two seasons, with the increasing rates of 25.6% and 20.3%, respectively.

Conclusions Straw mulching could alleviate the effect of soil temperature fluctuation, maintain soil moisture content to alleviate soil dryness. Straw mulching inhibits the seedling density of oilseed rape at the early stage. However, in the later period, straw mulching could maintain rape density, promoting growth, biomass, yield and nutrient absorption of winter oilseed rape.