Effect of freeze-thawing cycles on nitrogen and phosphor concentrations in black soil under different fertilization modes

In China, the freeze-thawing phenomenon occurs in the northeastern region, which happens in autumn, winter and early spring and can affect the structure as well as physical and chemical properties of the black soil. The concentrations and speciation of nutrition elements (nitrogen and phosphor) in black soil are also influenced by the freeze-thawing, which may have effects on the fertility of the soil or even result in eutrophication. In order to find out the change of nitrogen and phosphor concentrations and availability in black soil with various levels of fertilizers under freeze-thawing condition, a simulation freeze-thawing experiment was proposed in this paper. Various levels of inorganic nitrogen and phosphor fertilizers as well as organic fertilizer were applied in 10 maize plots based on the results of investigation on agricultural production in Heilongjiang Province and the specific circumstance of the present experiment. Then the black soil samples were collected from the 10 plots and put through simulative freeze-thawing cycles in the lab. After each cycle, the parameters of the soil samples were analyzed, including the soil properties (moisture contents, mass fraction of organic matters) and mass fractions of total nitrogen, available nitrogen, total phosphor and available phosphor. It was shown that freeze-thawing process could increase the moisture contents of soil samples for all the 10 treatments (P〈0.05). And under the freeze-thawing condition, the inorganic and organic fertilizers could improve the water holding capacity, resulting in higher moisture contents than the unfertilized soil (P〈0.05). The mass fraction of organic matters in soil samples for all the 10 treatments significantly decreased after 5 freeze-thawing cycles, and the highest reduction rate after 5 cycles was obtained in the sample with only organic fertilizer (29.7%, P〈0.05). For the samples with inorganic fertilizers, concentrations of total nitrogen decreased significantly after 5 freeze-thawing cycles (P〈0.05), indicating that freeze-thawing process could result in the loss of total nitrogen in black soil. The reduction rate of total nitrogen concentration after 5 freeze-thawing cycles was positively related to the quantity of inorganic nitrogen fertilizer (r=0.9899) when the level of inorganic phosphor fertilizer was constant. When organic fertilizer was applied, the total nitrogen concentrations in soil samples after 5 freeze-thawing cycles changed insignificantly, therefore the loss of total nitrogen could be inhibited. For all the samples except the one where inorganic nitrogen fertilizer level was the largest, concentrations of available nitrogen increased significantly after 5 freeze-thawing cycles (P〈0.05), while the application of inorganic nitrogen and phosphor fertilizers could inhibit the increase of available nitrogen concentration. The increasing rate of available nitrogen concentration after 5 freeze-thawing cycles was negatively related to the quantity of inorganic nitrogen fertilizer (r=-0.9675) when the level of inorganic phosphor fertilizer was constant. When the organic fertilizer was applied in the soil samples, the increasing rates of available nitrogen mass fraction were the highest among all the treatment (20.8% and 23.6%, P〈0.05), indicating that application of organic fertilizer could improve the availability of nitrogen in black soil. The concentrations of total phosphor in samples for all the treatments changed insignificantly after the freeze-thawing cycles. For all the 10 treatments, the concentrations of available phosphor in soil samples increased after 5 freeze-thawing cycles (P〈0.05). The application of either inorganic or organic fertilizer in the experiment could make the increasing rate higher, and the highest occurred when organic fertilizer were applied (23.4% and 27.1%, P〈0.05), suggesting that the availability of phosphor in black soil could be higher when organic fertilizer was applied. In conclusion, during the agricultural practice of black soil in cold regions under freeze-thawing condition, the quantity of organic fertilizer applied should increase appropriately in order to inhibit the loss of total nitrogen, as well as to improve the availability of nitrogen and phosphor, so as to maintain the fertility of black soil and lower the risk of eutrophication.