Ecological stoichiometric characteristics of soil nutrients and eco-enzymatic activities under different long-term fertilizations in a cinnamon soil

Objectives We studied soil element content, the enzyme activity related to C, N and P availability and their stoichiometric changes to understand the effects of long-term fertilization on the biogeochemical characteristics and nutrients cycling of farmland.

Methods The long-term field experiment was conducted in Shouyang County of Shanxi Province, and started in 1992. The study had nine fertilization treatments: no fertilizer control (CK), four chemical fertilizer treatments (N₁P₁, N₂P₂, N₃P₃, N₄P₄), three chemical fertilizer combined with manure treatments (N₂P₁M₁, N₃P₂M₃, N₄P₂M₂), and one manure treatment (M₆). Among these fertilization treatments, N₁, N₂, N₃ and N₄ represent urea N input rates of 60, 120, 180 and 240 kg/hm²; P₁, P₂, P₃ and P₄ represent super calciumphosphate input rates of 37.5, 75.0, 112.5 and 150.0 kg/hm²; M₁, M₂, M₃ and M₆ represent manure application rates of 22500, 45000, 67500 and 135000 kg/hm², respectively. In 1992, 2001, 2006 and 2016, soil samples were collected within 0–20 cm layer in all the nine treatments for the determination of soil organic carbon (C), total N (TN), total P (TP), available N (AN), available P (AP) content, and the activity of C-acquiring enzyme (β-1, 4-glucosidase, BG), N-acquiring enzyme (β-1,4-N-acetylglucosidase, NAG) and P-acquiring enzyme (alkaline phosphatase, ALP). The soil elements stoichiometric ratios (C∶TN, C∶TP, TN∶TP) and enzyme C∶N (lnBG/lnNAG), enzyme C∶P (lnBG/lnALP) and enzyme N∶P (lnNAG/lnALP) ratios were analyzed. The enzyme vector angles > 45° and < 45°, and vector length were induced to evaluate the resource limitation degree to microorganisms by C, N and P. The greater values of vector angle above 45° indicate stronger P limitation faced by the microorganism, vector angle under 45° mean that stronger N limitation, and larger vector length values indicate more severe C limitation of microbial metabolism.

Results 1) Except that soil C : TN was significantly higher under the N₄P₄ treatment than the M₆ treatment, there were no significant differences in C : TN and C : TP between the chemical fertilizer and manure treatments, while soil TN : TP was significantly lower (P<0.05) than M₆ treatment under other fertilization treatments. Compared to CK, M₆ treatment reduced the mean C∶TN value by 30.2% (P< 0.05), signicicantly increased the mean TN : TP by 41.0% (P<0.05), and N₄P₄ treatment reduced the soil C∶TP and TN∶TP by 23.3% (P<0.05) and 17.6%, respectively (P<0.05). The soil C∶AN (2.11–2.29) did not differ significantly among treatments, while the soil C∶AP and AN∶AP decreased with the increasing inputs of both chemical and organic fertilizers. Chemical fertilizer treatments decreased the C∶AP and AN∶AP less than the manure treatments did. 2) Soil lnBG/lnNAG, vector angle and vector length ratios were slightly higher (P>0.05) and lnNAG/lnALP ratios were slightly lower (P>0.05) than the manure treatments under long-term N₂P₂, N₃P₃ and N₄P₄ treatments. The lnBG/lnNAG was all higher but the lnNAG/lnALP was lower than their initials in 1992. The calculated vector angle was higher than 45° for all fertilization treatments, and the vector length was 11.1%–52.4% higher than those in 1992. 3) Significant correlations were found between AN and the activities of BG, NAG, ALP and the eco-enzymatic stoichiometries of lnBG/lnNAG and lnNAG/lnALP. There was no significant correlation between the contents of C, TP, AP and eco-enzymatic activities and stoichiometric ratios. Besides, soil C∶TN and C∶AN were positively correlated with lnBG/lnNAG (P<0.01), and TN∶TP positively correlated with lnNAG/lnALP (P<0.05). However, there was no significant correlation between lnBG/lnALP and all environmental factors.

Conclusions Long-term application of chemical fertilizers promoted soil organic carbon consumption and phosphorus fixation, and aggravated the carbon and phosphorus limitation of microbial metabolism. The application of organic fertilizer increased soil organic nitrogen, accelerated organic matter decomposition, but significantly increased available phosphorus. Thus, application of organic fertilizer alleviated resource constraints faced by microorganism, and effectively promoted elements turnover in cinnamon soil. In summary, changes in soil element stoichiometry under long-term fertilization treatments would cause non-steady changes in eco-enzymatic stoichiometry, as the expression of enzyme activities is strongly dependent on resources.