Introduction

The internal nitrogen (N) cycle in soil involves all processes that transform N from one chemical form to another and the transport between different N pools (Nannipieri and Paul 2009). These processes are driven by the abundance and activity of soil microorganisms (Stark and Firestone 1996; Shi et al. 2006), which can be affected by various factors, such as substrate quality and quantity (Nannipieri and Paul 2009), soil pH (Ste-Marie and Pare 1999; Pietri and Brookes 2008), and water content (Chen et al. 2011). Change in temperature is associated with the change in substrate affinity or ability to access substrates elicited by varying microbial activity or community composition (Nedwell 1999; Wallenstein and Hall 2012). Moreover, temperature is an important factor that affects N transformation in soils (Lützow and Kögel-Knabner 2009). However, the effect of temperature on N transformation processes is complicated because several biochemical processes with different time constants and different dependencies on other soil variables are involved. In addition, the effects of changing temperature on N transformation processes may be masked by simultaneous changes in the soil water content (Ye et al. 1994; Maag and Vinther 1996; Godde and Conrad 1999). Therefore, the influence of temperature on N cycles in different soils has to be investigated at comparable soil water contents.

The limited amount of information that can be acquired from net changes of inorganic N pools have increasingly been recognized because net changes are the result from several opposing soil processes (Nannipieri and Paul 2009). The gross rates of NH4 + and NO3 − production measured via the 15N isotopic pool dilution method provide more information about N cycling processes than the net rates (Murphy et al. 2003; Booth et al. 2005). The former allows us to assess separately the microbially mediated processes of production and consumption that simultaneously regulate N availability (Davidson et al. 1991). Gross N transformation rates need to be determined to quantify the response of each of those N cycling processes to temperature.

Studies on the influence of temperature on soil N transformation processes is extensive, but few of them investigated paddy soils (Zhang et al. 2007; Jin et al. 2012; Zheng et al. 2012; Zhu et al. 2013), especially at low temperature (<15 °C), a prevailing condition during winter and spring in different parts of the world (Andersen and Jensen 2001; Cookson et al. 2002). Rice paddy soil is a unique anthropogenic soil type formed under long-term hydroagric management with seasonal submergence, and it is especially important for the food production of China. However, the N conservation in paddy soil is difficult because of the wetting-drying water regime (Kundu and Ladha 1999; Fan et al. 2007). The current sustainability of rice cultivation is threatened by the low-use efficiencies of N fertilizer and heavy environmental impacts (Xing et al. 2001; Zhu and Chen 2002). A better understanding of the effect of temperature on gross soil N transformation processes in paddy soil will improve our ability to interpret and predict management effects on soil N availability and N loss to the environment.

The main objectives of this experiment are to examine the effects of temperature (range of 5 to 35 °C) on gross N mineralization, nitrification, and immobilization rate in paddy soils. Laboratory incubations were conducted in this study to avoid complications by plant uptake, leaching, and denitrification, despite the fact that sieving and moisture content manipulation could affect N transformation rates (Booth et al. 2005). The short-term (11 days) dynamics of the gross N transformation rates were estimated using the FLUAZ numerical model by using a double 15N label (in the form of 15NH4NO3 and NH4 15NO3) and employing the pool dilution principle (Mary et al. 1998). Gross mineralization and nitrification calculated by FLUAZ were compared with the results obtained using the classical isotopic dilution equation developed by Kirkham and Bartholomew (1954).

Materials and methods

Site description and soil sample collection

Soil samples were collected from our experimental fields under summer rice-winter wheat rotation before the wheat harvest from Dapu township, suburban Yixing County (31°17′N, 119°54′E), and from Lingqiao village, suburban Huai’an County (33°43′N, 118°86′E), situated in southern and northern Jiangsu Province, China. The average annual temperature in Yixing County was 15.7 °C, and average annual rainfall and evaporation were 1,198 and 1,230 mm, respectively. The average annual temperature in Huai’an County was 14.8 °C, and average annual rainfall and evaporation were 894 and 1,634 mm, respectively. The field experiment included three fertilization treatments [(i) “conventional” N fertilization (local farmers’ fertilization practice), (ii) reduced N fertilization, (iii) zero N fertilization] and conducted to determine the minimum N application rates to achieve the same yield of rice and winter wheat as local farmers achieved in the North and South Jiangsu (five repetition for each treatment). Soil samples used in this study were only samples from conventional N fertilization treatments plots of each experiment site.

The soil in Yixing developed from alluvial deposits was classified as hydragric anthrosol (HA), and the soil in Huai’an developed from lacustrine sediment was classified as anthraquic cambisol (AC) based on the World Reference Base for Soil Resources system (IUSS 2007). The soil HA is neutral with sandy loam texture, while the soil AC is alkaline with clay texture (Table 1). The original water content of soil HA and soil AC was 25.4 and 21.8 %.

Table 1 Key properties of the studied paddy soils
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Fifteen field–moist soil cores (top 20 cm of each soil) were collected from each site and pooled after the removal of litter. The remaining roots and leaf pieces were removed by hand. The fresh soil was ground to pass a 2-mm sieve, mixed to ensure homogeneity of fine soil, and stored at 4 °C for the incubation experiment.

Incubation and analytical procedures

A series of 250-mL Erlenmeyer flasks was prepared with 40 g (oven-dry equivalent) of fresh soil for each soil type. The flasks were covered with silicone plugs with small holes for aeration. Two milliliters of 15NH4NO3 or NH4 15NO3 solution was added to each of the flasks to provide an N concentration of 50 mg N kg−1 as NH4 + and 50 mg N kg−1 as NO3 − (three replicates for each 15N label treatment). The soil was adjusted to a moisture content of 60 % water holding capacity (WHC) and incubated for 11 days at 5, 15, 25, and 35 °C, giving a total of eight treatments. The flasks were sealed with silicone rubber stoppers. The samples were aerated by removing the stoppers for 30 min everyday. All the soil in the flasks from each treatment was extracted on sample date after 2 h, and then day 1, 2, 4, 7, 11. Throughout the incubation period, the moisture level was maintained at 60 % WHC. The loss of water was determined by weighing the flasks on alternate days, and corresponding amounts of distilled water were added as needed.

On each sampling date, soil in the flasks were shaken with 200 mL of 2 M KCl for 1 h at 200 rpm at 25 °C and filtered through qualitative filter paper. The filtrates were then stored at 4 °C to be analyzed for mineral N (NO3 − and NH4 +) concentrations and for their 15N abundance (Lan et al. 2013). The KCl-extracted soil was rinsed with distilled water to remove residual mineral N and oven-dried at 55 °C to measure the insoluble organic N concentration and its 15N composition (Lan et al. 2013).

Analytical procedures

The concentrations of mineral N (NO3 − and NH4 +) were determined using a continuous-flow auto-analyzer (Skalar, Breda, The Netherlands) and expressed as milligrams N per kilogram soil. The 15N enrichment of the NH4 + in the KCl extracts was determined by first generating ammonia (NH3) following the addition of MgO. The NH3 was absorbed by a H3BO3 solution, which was later dried to a residue. The 15N atom percentage of NH4 + was determined by measuring the 28N2/29N2 and 28N2/30N2 mass ratios via isotopic mass spectrometry (Finnigan, MAT 251, USA.) after the conversion of NH4 + to molecular N2 by using NaBrO. The determination of the 15N enrichment of the NO3 − in the KCl soil extracts was based on the production of N2O from hydroxylamine intermediates during reduction with Cd/Cu and described in detail in our previous research (Lan et al. 2013). The concentration and 15N composition of insoluble organic N remaining in the soil after KCl extraction were determined via mass spectrometry after converting NH4 + in soil to molecular N2 by Kjeldahl digests by using sodium hypobromite (Zhang et al. 2009).

Calculations

Nitrogen fluxes were calculated using the FLUAZ numerical model (view and see Mary et al. 1998 for details) by using the variables NH4 +, 15NH4 +, NO3, 15NO3 −, and organic-15N data collected from 15NH4NO3 and NH4 15NO3 treatment. The FLUAZ model method has several advantages over analytical methods (Mary et al. 1998): (i) the FLUAZ model simultaneously allows all rates and provides confidence intervals and a correlation matrix between parameters, and (ii) data variation is accounted in FLUAZ, and variables are weighted to avoid giving too much significance to variables with high coefficients of variability (Mary et al. 1998; Andersen and Jensen 2001). The main N transformations considered in FLUAZ are nitrification (n), mineralization (m), immobilization of NH4 + (i a) and NO3 −(i n), ammonia (NH3) volatilization (v), and denitrification (d) and re-mineralization of the newly formed microbial biomass (r) (Mary et al. 1998). Ammonia volatilization rate was assumed to be 0, the rates of remineralization were negligible (<0.5 mg kg−1 day−1), and d was undetectable because of the short-term aerobic incubation at 60 % WHC in our incubation experiment. The results are not shown in this paper. Furthermore, the gross rates of m and n calculated by FLUAZ were compared with the results obtained using the classical isotopic dilution equation developed by Kirkham and Bartholomew (1954)

$$ m=\frac{\left[\left({\mathrm{AT}}_1-{\mathrm{AT}}_2\right)/\Delta t\right] \log \left({\mathrm{AL}}_1{\mathrm{AT}}_2\right)/\left.{\mathrm{AL}}_2{\mathrm{AT}}_1\right)}{ \log \left({\mathrm{AT}}_1/{\mathrm{AT}}_2\right)} $$
$$ n=\frac{\left[\left({\mathrm{NT}}_1\hbox{-} {\mathrm{NT}}_2\right)/\Delta t\right] \log \left({\mathrm{NL}}_1{\mathrm{NT}}_2/{\mathrm{NL}}_2{\mathrm{NT}}_1\right)}{ \log \left({\mathrm{NT}}_1/{\mathrm{NT}}_2\right)} $$

where AL is labeled NH4 +; NL, labeled NO3 −; AT, total NH4 + (labeled + unlabeled); NT, total NO3 − (labeled + unlabeled); m is gross mineralization rate; n is nitrification rate; and Δt is an interval of time. Subscripts 1 and 2 denote the initial and final values of a pool at two consecutive sampling dates, respectively.

The measured net mineralization was calculated from the difference in the NH4 + plus NO3 − concentrations between the sample measured at the start and the sample measured at the end of incubation period. The measured net nitrification was the calculated from the difference in NO3 − concentrations.

One measure of temperature sensitivity is the Q 10 value, which is the rate of increase in a biological reaction when the temperature is increased by 10 °C (Fang et al. 2005). The Q 10 value was calculated using the equation of Kirschbaum (1995), which allows Q 10 to be calculated using incubations at any two temperatures in this study:

$$ {Q}_{10}={\left({A}_2/{A}_1\right)}^{10/\left({T}_2-{T}_1\right)} $$

where A 2 and A 1 are the mean transformation rates at temperatures T 2 and T 1, respectively.

Statistical analysis

All data were analyzed using the SPSS 18.0 statistical package. Means ± standard deviation are reported in the text. The effect of temperature on gross N transformation rates was assessed via one-way ANOVA of Tukey test with a significance level of 5 % because of the limited number of soils and the non-normal distributions of the soil variables measured. The differences in soil properties and rates between the two paddy soils were analyzed using a paired t test.

Results

Size and atom percentage 15N excess of the NH4 + and NO3 − pools

Two hours after the application of 15N-labeled solution, 1.95–3.07 and 10.8–16.4 mg N kg−1 of NH4 + in soil HA and soil AC, respectively, was not recovered by the extraction when incubated at 5 to 35 °C. Moreover, during the incubation period of 2 h to 11 days, the concentrations of NH4 + in all treatments decreased as the incubation time progressed (Fig. 1). However, the decrease patterns differed between two soils and temperature treatments. The added NH4 + was quickly consumed within the first 2 days of incubation at temperatures of 25 and 35 °C (Fig. 1a) in soil AC. This consumption was faster than incubation at 15 and 5 °C. However, the NH4 + concentration of all temperature treatments decreased linearly and slowly as the incubation proceeded in soil HA. The fastest consumptions of NH4 + in soil HA also occurred at 25 and 35 °C and the slowest at 5 °C. The NO3 − concentration increased more slowly at lower temperatures (5 and 15 °C) than at higher temperatures (15 and 25 °C) (Fig. 1b).

Fig. 1
figure 1

Dynamics of NH4 + and NO3 − pools in hydragric anthrosol (HA) and anthraquic cambisol (AC) at incubation temperatures of 5, 15, 25, and 35 °C and a water content of 60 % water holding capacity (WHC). Vertical bars indicate standard deviations

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In the 15NH4 +-labeled treatment, the 15N enrichment of the NH4 + pool decreased significantly at 15, 25, and 35 °C (P < 0.05, Fig. 2a). The decrease at 5 °C was not significant (P > 0.05, Fig. 2a). By contrast, the 15N enrichment of the NH4 + pool remained stable during the entire incubation when labeled with 15NO3 − in all treatments (Fig. 2b). The 15N enrichment of the NO3 − pool increased significantly during incubation regardless of the soil type and temperature treatments in the 15NH4 +-labeled treatment (P < 0.05, Fig. 2c). However, the 15N enrichment of the NO3 − pool decreased significantly during the incubation when labeled with 15NO3 − (P < 0.05, Fig. 2d).

Fig. 2
figure 2

15N enrichment of NH4 + and NO3 − pools in hydragric anthrosol (HA) and anthraquic cambisol (AC) labeled with 15NH4NO3 and NH4 15NO3 at incubation temperatures of 5, 15, 25, and 35 °C and a water content of 60 % water holding capacity (WHC). Vertical bars indicate standard deviations (a, c 15NH4NO3 treatment and b, d NH4 15NO3 treatment)

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Gross and net N mineralization rate

The gross mineralization rate (m) calculated using the FLUAZ model of each measurement interval is shown in Table 2. The changes in m were steady, and the values of m were very low (<0.4 mg N kg−1 day−1) at 5 °C in both HA and AC soils. However, the values of m at higher temperature treatments (15–35 °C) attained the highest values initially (2 h to1 day) and subsequently decreased towards the end of the incubation regardless of the soil type (Table 2). These results indicate that a flush of m might occur at the beginning of the incubation after the addition of 15N solution.

Table 2 Rates of gross N mineralization (m), nitrification (n), and immobilization of NH4 + (i a) and NO3 − (i n) for the five measurement intervals calculated using the FLUAZ model in hydragric anthrosol (HA) and anthraquic cambisol (AC). Values in brackets are standard deviations (n = 3)
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The time-weighted average values of m derived using FLUAZ closely corresponded to the values of m calculated analytically (Fig. 3a, R 2 = 0.9256). The difference of m rates occurred when the temperature was increased from 5 to 35 °C (Fig. 4a), with the calculated Q 10 values ranging from 1.63 to 1.88 in soil HA and from 1.56 to 2.59 in soil AC (Table 3). However, no statistical difference was established when the temperature was increased from 5 to 15 °C in both soils (P > 0.05, Fig. 4a). Furthermore, difference of m occurred between the two paddy soils when at 25 and 35 °C (Fig. 4a, P < 0.05 for both), whereas soils were comparable at 5 and 15 °C (P > 0.05, Fig. 4a). The measured net mineralization rates were negative in soil AC when incubated at 5 and 35 °C and in soil HA when incubated at 5 °C but positive in others (Fig. 4b).

Fig. 3
figure 3

Correlation between gross N mineralization and nitrification rates calculated using the 15N tracing model FLUAZ (FLUAZ, Mary et al. 1998) and analytical method (Kirkham and Bartholomew 1954)

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Fig. 4
figure 4

a–f Averaged rates of gross N mineralization (m), nitrification (n), immobilization of NH4 + (ia) and (NH4 ++NO3 −) (i), net N mineralization (m net), and net nitrification (n net) for intervals day 0–11, in hydragric anthrosol (HA) and anthraquic cambisol (AC) at incubation temperatures of 5, 15, 25, and 35 °C and a water content of 60 % water holding capacity (WHC). The different letters above the bars indicate significant differences between temperature treatments with same soil at P < 0.05. Vertical bars indicate standard deviation. n = 3 (Note m, ia, i, and m net were time-weight averaged rates, but both n and n net are the NH4 + concentration-weighted average rate)

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Table 3 Temperature coefficient (Q 10) values of gross N transformation rates of mineralization (m), nitrification (n), and immobilization of NH4 + (i a) and (NH4 ++NO3 −) (i) in hydragric anthrosol (HA) and anthraquic cambisol (AC) at incubation temperatures of 5 to 35 °C
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Gross and net N nitrification rate

The modeled gross nitrification rates (n) of each measurement interval increased with temperature from 5 to 25 °C in both HA and AC soils. However, the rates were similar at 25 and 35 °C (Table 2). The changes in n in soil HA at the four temperature treatments remained nearly constant during the course of incubation (Table 2). The values of n were consistent with the fast consumption of NH4 + in soil AC. Moreover, n was extremely high during the first 2 days at 25 and 35 °C and during the first 4 days at 15 °C (>10 mg N kg−1 day−1), but it became very low (<1 mg N kg−1 day−1) with the depletion of NH4 + (Table 2) in soil AC.

The NH4 + concentration-weighted averages of gross and net nitrification rates are shown in Fig. 4c, d, respectively. The average values of gross n derived using FLUAZ also closely corresponded to the values of n calculated analytically (Fig. 3b, R 2 = 0.9351). The average gross n ranged from 2.21–6.52 mg N kg−1 day−1 in soil HA and from 3.53–21.3 mg N kg−1 day−1 in soil AC. The calculated Q 10 values ranged from 1.12 to 2.25 in soil HA and from 0.99 to 2.85 in soil AC. The Q 10 values generally tended to decrease with increasing temperature in both soils (Table 3). The net nitrification rates (n net) were positive in all temperature treatments (Fig. 4d). The trends of soil type and temperature on gross nitrification rates were similar to that of the net nitrification rates (Fig. 4c, d), but the n net were generally lower than the corresponding gross n. Furthermore, the average gross and the net nitrification rates in soil AC were significantly higher than those in soil HA when incubated at the same temperature (P < 0.05).

Gross N immobilization rates

Based on the results from the FLUAZ model, we determined that NH4 + immobilization (i a) occurred in HA and AC soils at the four temperature treatments. However, no NO3 − immobilization (i n) occurred in the two soils at 5 °C, a small amount of NO3 − immobilization at 15 °C, and an increasing amount of NO3 − immobilization when the temperature was increased from 25 to 35 °C as long as a large amount of NO3 − is accumulated in the two soils (Table 2). The proportion of i a ranged from 55 to 100 % of the total mineral N (NH4 + + NO3 −) immobilization (i) at all time intervals investigated in the tested soils. These results indicate that NH4 + was the main form consumed by the microbes in the soils tested. In both HA and AC soils, the values of i a were generally higher during the first 2 days after the addition of NH4NO3 solutions (Table 2).

The average values of i a and i were temperature dependent and significantly increased when the temperature was increased from 5 to 35 °C (P < 0.05, Fig. 4e, f), only with the exception of i a in soil HA at temperatures from 15 to 35 °C which did not increase significantly (Fig. 4e). Moreover, the calculated Q 10 values of immobilization for a temperature interval 5 to 15 °C were higher than others (Table 3). Higher mineral N immobilization rates were observed in soil AC than in soil HA when incubated at 25 and 35 °C, whereas the rate differences between the two soils at low temperatures (5 to 15 °C) were not significant (P > 0.05).

Discussion

Temperature dependence of gross N transformation rates

Temperature is a key factor that regulates many terrestrial biogeochemical processes, such as soil respiration, litter decomposition, and N cycle. However, the effect of temperature fluctuation on N cycle were influenced by numerous factors such as temperature zone, land use, and soil texture (Rustad et al. 2001; Dessureault-Rompre et al. 2010). A recent metanalysis indicated that temperature sensitivity of N transformation may be greater in soils originating from colder climatic zones (mean annual temperature <2 °C, Q 10 = 3.33, n = 26) compared with warmer climate zones (6–14 °C, Q 10 = 2.13, n = 23; >14 °C, Q 10 = 1.99, n = 18) (Dessureault-Rompre et al. 2010). Besides, there was also a greater temperature response of soil N mineralization rate for agricultural compared with forested soils. Among agricultural soils, sand-loam soils had a greater temperature response compared with clay soils (Rustad et al. 2001; Dessureault-Rompre et al. 2010). In present study, consistently, the calculated Q 10 values of gross m, n, and i were also higher at low temperature than high temperature (Table 3). Furthermore, a general tendency of increasing N transformation rates with increasing temperature (5 to 35 °C) was observed in the tested paddy soils and was consistent with previous findings (Cookson et al. 2002; Hoyle et al. 2006). However, there are reports that no differences were observed in net nitrification or N mineralization rates at lower temperatures (−10, 0, and 5 °C), whereas significant differences were observed at higher temperatures (15, 25, and 35 °C) in grassland soils (Wang et al. 2006). Moreover, nitrification rate declined when the temperature was increased from 15 to 20 °C in particular cases (Stottlemyer and Toczydlowski 1999). In the current study, the sensitivity of different N transformation processes to temperature fluctuations differed. Low temperatures (5 to 15 °C) did not affect gross N mineralization rates (Fig. 4a) but significantly affected average gross N immobilization and gross nitrification rates (Fig. 4). This result is consistent with previous studies by Andersen and Jensen (2001) and Lang et al. (2010). They reported that the gross immobilization (i a + i n) process may be very sensitive to temperature change, whereas the gross mineralization process seemed to be relatively unaffected at temperatures <15 °C. Nitrification has also been expected to be more sensitive to temperature change than mineralization (Van Scholl et al. 1997) because nitrite oxidizers appear to be more sensitive than NH4 + oxidizers at low temperatures (Tyler and Broadbent 1960). In the present study, the optimum temperature for both gross and net N nitrification was around 25 °C (Fig. 4c, d), indicating that the activity of nitrifying bacteria in the tested paddy soils is optimal at or approximately 25 °C. However, evidence shown that the temperature optima for growth and enzyme activity are often greater than in situ temperatures in their environment (Stark 1996; Wallenstein and Hall 2012).

Differences in responses to temperature change between two paddy soils

The two paddy soils studied were developed from different parent materials and exhibited different soil pH and clay. The responses of gross N mineralization, nitrification, and immobilization rates to temperature change, as indicated by the Q 10 value (Table 3), were higher in soil HA than in soil AC when the temperature was increased from 5 to 15 °C. Nitrogen transformation in soils with fine texture were not affected by soil temperature (McInerney and Bolger 2000; Griffin et al. 2002), probably due to the formation of organic–inorganic complexes and soil aggregates and the protection of organic compounds from biological degradation. However, the above effect was undetectable at temperatures from 15 to 35 °C (Table 3). Besides, the possibility that the disruption of the soil aggregates caused by sieving, which may partly reduce the protection effects of clay on N transformations, cannot be excluded.

Significantly higher average values of m were detected in soil AC compared with those in soil HA at high-temperature treatments (25 and 35 °C, P < 0.05, Fig. 4a). The ratio of C/N is usually regarded as the most important regulator of N mineralization (Randlett et al. 1996). However, the C/N ratio is unlikely the cause of rate differences in gross N mineralization because the C/N ratio of the two soils was similar (Table 1). Previous studies have documented that soil pH and clay content are also important factors that affect the microbial turnover of soil organic matter but has an opposite function in soil organic mineralization. Clay has been shown to protect organic matter against decomposition (Schjonning et al. 1999; Cote et al. 2000; Müller and Hoper 2004; Kemmitt et al. 2006 ), whereas high soil pH can stimulate the mineralization of soil organic matter (Adams and Adams 1983; Kemmitt et al. 2006; Bertrand et al. 2007). Soil AC had higher clay content higher than soil HA, but we deduced that the stimulating effects of a high soil pH probably overwhelmed the protective effects of clay in the mineralization of organic N at higher temperature (25 and 35 °C). The Huai’an region (where AC soil was sampled) in the North Jiangsu Province belongs to the North China Plain both climatically and geographically. A previous study has also shown that calcareous soils in northern China Plain possess a higher N mineralization potential compared with those in the South (Roelcke et al. 2002). However, the above effect was not obvious at temperatures <15 °C in our study.

Furthermore, the gross and net nitrification rates were significantly higher in soil AC than in soil HA (P < 0.05, Fig. 4c, d) regardless of the incubation temperature. Nitrification is sensitive to soil pH (De Boer and Kowalchuk 2001), and the rate increases with pH values from 6.0 to 8.5 with the optimum at approximately 8.5 (Sahrawat 2008). Partial effect may be due to the presence of carbonates found at higher pH values (Schmidt 1982; Kinsbursky and Saltzman 1990). Carbonates can provide a source of CO2 needed for the growth of the autotrophic organisms involved. Therefore, the higher pH in soil AC (pH = 8.3) was most likely a key factor that resulted in its higher observed gross and net nitrification rates compared with those in soil HA (pH = 6.2) .

Implications of different N transformation rates in two paddy soils

Mineralization is a very important process of transformation of organic N into inorganic forms of NH4 +. Net N mineralization rate can be used to predict soil N supplying capacity. However, net N mineralization is the outcome of two concurrent and oppositely directed processes, namely, gross N mineralization and gross mineral N immobilization. Consequently, net mineralization rates do not clearly explain the total rate of microbial activities. Thus, an improved understanding of gross N mineralization and gross mineral N immobilization ratio (mineralization and immobilization M/I) can potentially improve our capability to predict net N mineralization patterns (Luxhøi et al. 2006). The calculated results show that the ratios of M/I in both AC and HA soil were <1 at 15 °C for both soils and at 25 °C for soil AC, which suggests that immobilization was the dominant process compared with mineralization. However, mineralization was dominant compared with immobilization in the other treatments. No significant differences were observed in the M/I ratios between the two soils at temperature higher than 5 °C. This result indicates that the N supplying capacity of the two paddy soils were generally similar at temperature higher than 5 °C.

Microbial nitrification and immobilization of NH4 + are two consumption processes of NH4 + in soil. The ratio of gross nitrification to gross ammonium immobilization (N/IA) rate could be used as an effective indicator for assessing the relative importance of the two consumption processes (Hoyle et al. 2006). The calculated results show that the N/IA ratios were also higher than 1 in all soil types and temperatures. These results indicate that the mineralized or fertilized NH4 + in the two soils was mostly consumed through nitrification. The N/IA ratios in soil AC were lower than in HA soil at 5 °C, whereas they were 1.5 to 3.2 times of that in soil HA at a temperature range of 15 to 35 °C. Therefore, a higher NH4 + consumption capacity by nitrification process at a high temperature was observed in soil AC than that in HA.

Although, great differences exist between the laboratory conditions of aerobic incubation and in situ conditions, including the stimulated consumption rates by substrate addition, changed soil water content, and changed soil aeration in laboratory study. However, what can be gained by laboratory studies is to get insights into the mechanisms behind N transformation. In field practice, Nitrification converts the relatively immobile mineral N form NH4 + into highly mobile NO3 − after the application of NH4 +-based fertilizers. Furthermore, nitrification is regarded as the most important direct driver of N loss during aerobic upland cropping season (Chen et al. 1998; Malla et al. 2005) and an indirect driver of N loss during flooded rice-growing season (Zhou et al. 2012). A higher nitrification capacity in soil AC might lead to the fast accumulation of NO3 −, and if the accumulated NO3 − is not absorbed by crops in time, they might be susceptible to losses following flooding of the fields for rice cropping at high-temperature conditions (Roelcke et al. 2002). In conclusion, the N dynamics in paddy soils were temperature dependent. However, the sensitivity of different N transformation processes to temperature fluctuations differed. A higher pH in alkaline paddy soil was regarded as the main factor for the higher gross N mineralization and nitrification rates. This condition probably raises a higher risk of NO3 − loss in field. The incubation moisture of 60 % WHC in our study was quite different from the native habitat of the rice-paddy soils, which might result in differences in the relationships between temperature and N transformation rate. Further study to confirm our results considering the interaction of temperature and water content on N transformation processes should be conducted. Future work should include an increased soil sample number derived from different parent materials to more definitively establish the factors influencing gross N transformation rates in paddy soils of China.