Study on different potato continuous cropping ways on rhizosphere soil nutrients and enzyme activities

To address the problem of food security, China produced potatoes as a staple food in 2015.However, there are increasing problems with continuous cropping production methods, potato continuous cropping has been inevitable.So it is necessary to research under the different potato continuous cropping ways, potato rhizosphere soil nutrients and enzyme activities which can direct potato fertilizer and ease potato continuous cropping obstacle. A two-growing season investigation was carried out during the spring and autumn of 2014 and 2015 to determine the different ways of potato continuous cropping on the overall growth of potatoes, soil nutrients, and enzyme activities. During continuous cropping nitrogen (N) content of rhizosphere soil was reduced; available potassium (Kav) was significantly reduced(p≤5%), especially in spring and autumn continuous cropping; and total phosphorus (Ptot) was reduced during the growth stage. However, the total potassium (Ktot), available phosphorus(Pav), and organic carbon (Ctot) increased before they decreased. For rhizosphere soil enzyme activities, urease initially increased and then decreased, and was lower in continuous cropping than multiple continuous cropping; in spring of 2015, invertase was the highest with continuous cropping. Catalase and polyphenol oxidase decreased initially before increasing. Continuous cropping in spring and autumn consumed more nutrients, especially potassium (K) than in spring. Therefore, potatoes planted in both spring and autumn enhanced the problems of continuous cropping. However, multiple continuous cropping that eased rhizosphere soil nutrient absorption and effectively improves soil nutrients and enzyme activities could provide an effective method for managing the negative impacts associated with continuous cropping.


INTRODUCTION
Follow the rice, wheat and maize, potato (Solanum tuberosum L.) is the fourth-most impo rtant food crop world wide. It is one of the majo r crops in Ch ina as well. To date, the planting acreage and production of potato in Ch in a ranked as top one worldwide. In 2015, the potato staple strategy are put forward in our country, by advancing the potato food development, adjusting measures to local conditions enlarges planting area, from the current 5.3*10 4 hm 2 to expand to 1.0*10 6 hm 2 (Schirring, 2015). However, due to the limitat ions of different crops in different ecological conditions and authentic, and optional rotation crop limitations such as low efficiency and the urgency of the economic demand, makes the potato continuous cropping pattern is inevitable.
In Gansu, Sichuan and Inner Mongolia, due to rapid development of potato industry, its planted area extended to more than 65% of the country (Long et al., 2013), the potato continuous cropping patterns have been inevitable under the condition of limited arable land. under the condition of several years of p lanting potato model continuously has inevitable. Especially with the co mparative benefits the improvement of potato industry, potato planting area in the northwest China increases year by year. Inding xi area, Gansu province, the potato continuous cropping has amounted to more than five years in some part of arable land, the potato continuous cropping is common, thus resulting in increasing problems for effective agricu ltural management in China. These problems include an increase of pests and diseases (Jeffrey et al., 2015), and a negative impact on soil physiochemical properties, soil microbes, and allelopathy (Rice,1985;Zhang et al., 2015). Previous researchers reported that continuous cropping decreases yield, quality, and causes diseases and imbalances of soil ecology (Wu at al., 2009;Zhang et al., 2010); thus, seriously affecting the production and economic benefits of potatoes.
It is worth mentioning that with development of China, the potato continuous cropping is inevitable. So explore the different ways of continuous cropping patterns, research rhizosphere soil nutrient and enzyme activity is significant.
The negative impacts of continuous cropping have become a major constraint in potato cultivation . The negative impacts of continuous cropping were mainly caused by an increase of soil-borne d iseases (Robert et al., 2014;Lai et al., 2011), the degradation of soil physical and chemical properties (Liu et al., 2009;Zhang et al., 2007), and root secretion and residue decomposition resulting in autotoxicity ( Although at present there have been lots of research about the potato continuous cropping obstacle, but the potato production practice of continuous cropping is not practically solved. This study based on the potato continuous cropping, under the different modes of the potato continuous cropping research onrhizosphere soil nutrient and enzyme activ ity of comparative analysis, aimed at the production of continuous cropping potatoes offer ideas and solutions.The objectives of this study were 1) to evaluate the effect of the continuous potato cropping on rhizosphere soil nutrients and enzyme activities on the growth characteristics of potatoes; and 2) to show the negative impacts of potato continuous cropping, direct the potato production. The results fro m the present study should provide a greater understanding of the negative impacts of continuous cropping and contribute to the improved management of potato cultivation.

Site descriptions
The experiment was conducted in 2014 and 2015 in the field at the experimental station in the college farm, southwest Sichuan Agricultural University, Chengdu, Sichuan Province (southwest China, N 30° 67 ', E 104° 06'). The soil chemical characteristics were: pH (1:1 water) 5.08, organic carbon (Ctot) 18.72 mg/ kg, total n itrogen(Ntot) 2.68 g/kg, total phosphorus (Ptot)0.58 g/kg, total potassium(Ktot) 13.00 g/kg, available nitrogen(Nav) 138.68 mg/kg, available phosphorus(Pav) 18.72 mg/kg, available potassium(Kav) 126.31mg/kg. The plant site before the experiment had been previously planted with a potato crop in 2013. The present study included A (mult iple continuous cropping), B (potato continuous cropping in spring), and C (potato continuous cropping in spring and autumn) cult ivation.All three cropping were conducted in a randomized b lock design of three repetitions in two years and two gro wing seasons. About 30 g whole potato tubers, cultivar Chuanyu 117, were planted at a depth of 8 cm.Six potatoes were planted into the compart ment with control soil fro m each pot (60 × 40 × 35 cm). Before potato planting, experimental plots were uniformly tilled and a compound fertilizer 750 kg ·ha -1 (N-P2O5- K2O 15-15-15) was init ially used as a base fertilizer. Fert ilizer was not added again. Standard agronomic practices were equally perfo rmed in all treatments. The sampling periods were before seeding, flowering, tuber bulking, and maturity. In autumn 2014 and 2015, the land of A and B was idle, and C was planted with potatoes (its index was not measured because planting in autumn was for enhancing the negative impacts of continuous cropping).

2.2Rhizosphere soil nutrients
The initial soil samples were g round by mortar and pestle to pass a 0.15 mm sieve to remove plant residues and stones prior to chemical analysis. Total nitrogen(Ntot), phosphorus (Ptot), and potassium(Ktot) were measured using the method described by Bao (2005).Soil organic carbon (Ctot) was determined using the Walkley-Black method; total nitrogen(Ntot) was assayed using the Kjeldahl method;total phosphorus(Ptot) was measured by phosphomolybdate-blue spectrophotometry; total potassium(Ktot) was measured by NaOH mo lten-flame photometry; organic matter was measured by heat dilution using potassium d ichro mate volumetry; alkali-hydrolyzab lenitrogen(Nav) was measured by alkali-hydroly zed reduction diffusion; available phosphorus(Pav) was measured using 0.5 mo l/ L NaCO3; and available potassium(Kav) was measured by NH4OAc leaching and flame spectrophotometry. 2.3Rhizosphere soil enzyme activities Soil enzy me analysis was performed on the same samples that were air-dried,ground, passed through a 1-mm mesh, and maintained at roo m temperature (Guan, 1989). Invertase activity was determined using 3,5-d initrosalicy lic acid colorimet ry; urease activity was measured using phenol-sodium hypochlorite sodium colorimetry; polyphenol oxidase was measured using phenol colorimetry; and catalase was assayed using a titrat ion method described by Guan (1989).

2.4Statistics analysis
The analysis were carried out using Microsoft Office Excel 2007, variance for the data was performed by DPS. Means were separated by LSD at the 0.05 probability level.

Nitrogen contents of rhizosphere soils
In 2014, co mpared with before seeding, in maturity stage, the content of totalnitrogen(Ntot) in rhizosphere soil was reduced by 4.41, 14.55, and 14.71%, for A1, B1, and C1, respectively. However, except for C2, the content of totalnitrogen(Ntot) varied smoothly in spring of 2015, the A2 was reduced by 1.01%, and B2 and C2 increased by 0.51 and 16.16%, respectively. The C content decreased with continuous cropping years. In maturity stage of 2014, the alkali-hydrolyzab lenitrogen(Nav) content reduced by 22.74, 26.10, and 20.26%, for A 1, B1, and C1, respectively; and in A2 and B2 increased by 28.89, and 18.46%, however, C2 decreased by 9.15%. In tuber bulking stage, the content of alkali-hydrolyzab lenitrogen(Nav) changed rapidly. Alkali-hydrolyzab lenitrogen(Nav) content showed little difference in different continuous cropping ways in the spring of 2014, and were reduced in 2015. The variation of totalnitrogen(Ntot) showed the opposite trend. 3.2Phosphorus contents of rhizosphere soils Fro m table 2, total phosphorus(Ptot) was decreased with growth as the continuous cropping years increased. These three treatments showed the significant effects on the total phosphorus(Ptot) . A1 increased by 3.32%, and B1 and C2 were reduced by 15.49 and 14.53%, respectively. In 2015, the content of total phosphorus(Ptot) was reduced by 13.11, 23.07 and 15.71% fo r A2, B2, and C2, respectively. Throughout the growth stages, there was a considerable change in total phosphorus(Ptot) content. In maturity stage, the total phosphorus(Ptot) content ranged fro m 0.500 to 0.600 g/kg. The variation of available phosphorus(Pav) content is different fro m total phosphorus(Ptot). In spring of 2014, available phosphorus(Pav) was reduced by 46.99, 39.56, and 39.42% for A1, B1, and C1, respectively. In contrast to that in 2015, it increased by 28.35, 25.03, and11.96% for A2, B2, and C2, respectively. Even though the total phosphorus(Ptot) in the whole growth period fluctuated largely, its content did not significantly reduce. In the spring of 2014, phosphorus content increased, total phosphorus(Ptot) was insignificantly different fro m mu ltiple continuous cropping, and the available phosphorus(Pav) content was more reduced than multiple continuous cropping in the spring of 2015. 3.3Potassium contents of rhizosphere soils As for total potassium(Ktot), in maturity stage, there was minimal difference between each treatment. Total potassium(Ktot) increased before it decreased. Eventually, the contents were reduced by 5.39, 1.85, 1.57%, for A 1, B1, and C1, respectively, and then increased by 4.00, 4.37, and 0.00% for A 2, B2, and C2, respectively. Available potassium(Kav) was reduced by 5.54, 23.92, and 24.46% for A1, B1, and C1, respectively, but thereafter increased by 30.55, 126.47, and 21.02% fo r A2, B2, and C2, respectively. Fro m Table 3, total potassium(Ktot) content in rhizosphere soil was relat ively stable, but the available potassium(Kav)content varied greatly.potato continuous cropping in spring and autumn was lower than mu ltiple continuous cropping year by year. Potatoes, as a high potassium crop, required mo re potassium for self-growth. In present study, potato continuous cropping consumed a small amount of total potassium(Ktot), but large amounts of available potassium(Kav). Th is can directly affect the application of fertilizer in production.  3.5 Enzyme activities of rhizosphere soils Table 5 showed that urease steadily increased in t 2014, with the maximu m level in flowering stage. In maturity stage, it increased by 34.00, 22.37, 21.71%, for A1, B1, and C1, respectively. and it increased by 20.33, 9.24, and 5.83%, for A2, B2, and C2, respectively. Although over both years urease increased, it was lower in 2015 than that in 2014.The rhizosphere soil invertase activity in maturity stage increased by 52.07, 7.65, 11.24%, for A1, B1, and C1, respectively.In spring of 2015, A2, and B2 in maturity stage decreased by 52.92 and 19.39%, respectively, whereas C2 increased by 25.47%. For catalase, B1 and C1 were reduced by 13.68 and 8.62%, whereas A1 increased by 13.86%. In 2015, catalase increased by 9.88, 6.13 and 11.88%, for A2, B2, and C2, respectively. In present study, there was little difference between the treatments in 2014, with only small variations. In maturity stage, A1 increased by 3.67%, and B1 and B2 were reduced by 10.05 and 1.66%, respectively. However, in 2015, polyphenol o xidase activity chan ged considerably, especially in tuber bulking stage. Eventually, the activity of polyphenol oxidase increased by 27.97, 25.28, 5.24%, for A2, B2, and C2, respectively. IV. DISCUSSION It is generally hypothesized that the soil nutrients decreased annually with continuous cropping (Xiao et al., 2012). But in this research, the available phosphorus(Pav),the available potassium(Kav) contentand organic carbon (Ctot) content were different. This illustrated that the different crops, the difference of continuous cropping ways and seasons, the content of potato root nutrients, and root secretions fro m the to xic effect and residues, had different effects on soil nutrients. When plant roots and s oil are in close contact, a large variab ility of soil area is formed, and the micro-ecological environ ment of the potato rhizosphere soil changes. Therefore, under the condition of constant continuous cropping, this inevitably affects plant growth and development.
Soil enzy me activit ies play an important role in the soil ecosystem, and can reflect the changes in soil quality. So il enzy me activit ies are the key to overco me the problems associated with continuous cropping and other agricultural practices that destroy the soil health (Yim et al., 2013). In the present study, compared with potato multip le continuous cropping, the activities of urease, polyphenol oxidase, and invertase were lo wer in the spring of 2014 but higher in 2015. Catalase activity was lower and then little different. Simu ltaneously, as for continuous cropping years, urease enzy me activity was reduced, and invertase,catalase, and polyphenol oxidase activities increased. In tuber bulking and maturity stage, rhizosphere soil enzy me activit ies increased significantly. During the tuber-bulking and maturity stage, the potato root system shows high activity, utilizing excessive rhizosphere soil nutrients, which increases the rhizosphere soil enzyme activity.
Polyphenol oxidase (PPO) is secreted by the plant roots, increasing soil microbial act ivity and the decomposition of plant and animal residues, releasing co mpound enzymes (Claus et al. 2010;Yoruk et al., 2003). These are biodegradable soil phenolics that slow down the allelopathy between plants, thereby creating advantageous conditions for plants to expand their habitats. In the present study, rhizosphere soil polyphenol oxidase activity increased with increasing years of planting. The research results are similar to the research on Caraganakorshinskii (Cao et al., 2008), where polyphenol oxidase activity of continuous cropping in spring and autumn is higher than that in spring. The soil enzy me activ ity of sucrase, urease, and catalase has been related to soil fert ility, butonly less data is available on the activity of polyphenol oxidase. In the present study, there were significant differences in polyphenol o xidase activity under continuous cropping. Therefore, soil polyphenol oxidase can be considered as an indicator of the negative impact of continuous potato cropping, but requires further verification.
As for the result of the tuber yield, there is a certain relationship between the tuber yield and continuous cropping years, the method of cult ivation pattern. But a small difference soil change between mult iple continuous cropping(A) and potato continuous cropping in spring (B) andcontinuous cropping in spring and autumn (C) in the short term of continuous cropping for two years, there is a significant difference between them for continuous cropping over three years. Potato continuous cropping consumed more nutrients, especially available nutrients. We think potato continuous cropping for two years is possibly the threshold for the local ecological environmental conditions and cultivation. For potato growing, to meet production requirements and the economic benefits of continuous cropping, the most efficient cu ltivation period for the process would be two years, and plans to extend it would require a reasonable arrangement to provide a practical basis. Meanwhile, continuous cropping in spring and autumn