Results and discussion Directory UMM :Data Elmu:jurnal:E:European Journal of Agronomy:Vol12.Issue3-4.Jun2000:

3. Results and discussion

3 . 1 . Nitrogen supplied with fertilization and N remo6al Table 4 reports the amounts of N supplied as fertilizer and N removal at harvest in the eight examined cultivation systems, as an average of 2 years, and the corresponding variability. The four indicators of the fertilizer efficiency are also re- ported in the table. The total amount of N fertilizer supplied by the farmers was remarkably different over the cultiva- tion systems. The permanent meadows CSa 1 and CDa 1 received approximately 130 kg N ha − 1 year − 1 , the lowest fertilization level of all the crops. N was supplied mainly as cow manure, which was spread in winter and spring, after the first cut. N removal exceeded fertilization, expe- cially at CSa 1 , maybe because of the N fixation, as the contribution of white clover to the sward composition was about 12. The N apparent recovery in the meadows ranged from 19 to 30. In all the other situations, where annual crops were cultivated, the total N inputs were remark- ably high, ranging from 369 to 509 kg N ha − 1 year − 1 , as has frequently been recorded in the Po plain Borin et al., 1997. In general, organic fertilizers were applied just before soil tillage, in spring before maize or in autumn before winter cereals. Only at PSb, in both years, was pig slurry spread partly in autumn and partly in spring, with spring ploughing. Another exception was maize at PD. In the first year slurry was in fact supplied in autumn 1994, and in spring 1996 in the second year, and, in both cases, ploughing followed in 1 – 2 days. The high fertilization levels in all the cropping systems resulted in a calculated surplus of 128 – 335 kg N ha − 1 year − 1 . The N removal- fertilization ratio ranged from 0.32 to 0.65, which is consistent with what was reported by Smith et al. 1994, while only the highest value corre- sponded to what was measured by Vetter and Lorenz 1990. The apparent recovery index showed a high variability between 0 and 26, which corresponded to a N use efficiency index that ranged between 1 and 13 kg of the total DM kg N − 1 . Schro¨der and Ten Holte 1994 reported higher apparent N recovery values about 10 – 39, on silage maize fertilized with manure, as did Bocchi and Tano 1994, who found that manure fertilization produced 3 – 14 kg of maize grain per kg of manure-N. The low values of all N efficiency indexes, also compared with literature values, have demon- strated that the examined arable crops were over- fertilized. The accumulation of nitrogen over the years led to a build up of a high soil mineraliza- tion potential, as indicated by the unfertilized plots, where the plant uptake was 117 – 209 kg N ha − 1 year − 1 in the meadows, and ranged from 95 to 272 kg N ha − 1 year − 1 in the arable crops. The maize generally tolerated the N surplus, with the exception of site PSb, where even negative appar- ent recovery indexes were recorded. The results shown in Table 4 show that N fertilization was not modulated on the crop up- take or the crop removal, as frequently occurs in intensive livestock farming systems. The necessity of disposing of excreta cannot entirely explain the large recorded N surplus. In three situations out of four CS, CD and PS mineral fertilization, in fact, greatly contributed to a build up of the N surplus. Only at PD did the farmer efficiently use slurry also to top dress maize, thus reducing the mineral N input. When compared to maize as a single crop, the combination of summer and winter crops did not modify the N removal-fertilizer ratio: at PS, be- cause both the removal and fertilizer input were similar in both situations, and at PD, because they both increased compared to maize as a single crop. The amount of fertilization supply showed a remarkable variability, which was higher than what was measured for the crop removals. This confirmed an arbitrariness by the farmers as far as fertilization is concerned. The data presented in Table 4 can be better analyzed on the basis of the relationships pro- posed in Fig. 2. These indexes are among the most used to evaluate the sustainability of crop with regard to the use of fertilizers. The calculated N surplus does not take account of the N fluxes in the soil i.e. organization, mineralization or variation of the soil total N, C . Grignani , L . Za 6 attaro Europ . J . Agronomy 12 2000 251 – 268 259 Table 4 N fertilization, N removal and N use indexes for the different cultivation systems. All data are averages of the 2 year period a Removal Surplus Removal-fertilizer N use efficiency Apparent recovery Cultivation system Mineral fertiliz. Organic fertiliz. kg ha − 1 kgN 100kgN − 1 kg ha − 1 kg ha − 1 ratio kg ha − 1 kgD.M. kgN − 1 − 118 1.92 30 30 247 a 1 CS 34 95 Avg. 3.6 11.9 0.10 4.0 6.8 Std. Err. 14.3 6.9 264 b 163 0.62 13 26 Avg. 272 155 17.4 0.03 5.2 6.1 9.6 Std. err. 25.0 8.7 a 1 142 − 12 1.10 19 19 Avg. 94 CD 36 4.2 15.7 0.20 4.0 8.7 Std. err. 14.0 4.6 255 0.32 10 15 120 146 229 Avg. b 2.9 16.2 93.0 0.14 2.7 4.5 Std. err. 86.3 335 0.34 7 15 116 PS 174 394 Avg. a 2 20.9 58.0 0.07 0.6 1.9 Std. err. 59.7 19.3 183 b 303 0.38 1 Avg. 341 145 62.3 0.05 1.0 3.1 8.9 0.0 54.8 Std. err. 242 a 2 128 0.65 3 16 Avg. 297 74 PD 67.1 0.11 6.0 11.9 33.3 56.0 30.3 Std. err. 197 B 171 0.54 4 17 Avg. 358 11 6.4 25.6 11.2 0.03 3.6 6.3 Std. err. 37.9 a The standard error shows the sum of spatial between the two plots and temporal between the two years variability. CS, cattle shallow, CD, cattle deep, PS, pig shallow, PD, pig deep. The cultivation systems a 1 , a 2 and b are described in Fig. 1. but it is the most simple way to approach the N balance at the field scale, and therefore it is very suitable to judge fertilization management. The calculated N surplus increased linearly as the fer- tilization input increased R 2 = 0.89, with a break-even point at about 200 kg N ha − 1 and a slope equal to one. If one excludes meadows CSa 1 and CDa 1 , where nitrogen fixation altered the N surplus calculation, the equation did not change in practice y = 1.05x + 216.92: R 2 = 0.66. Well-managed crop systems should show both a low N surplus and an independence of the calcu- lated surplus from the fertilization level. The same variables used to calculate the surplus index can be combined in the removal-fertilizer ratio. As expected, the two indexes were closely and inversely correlated R 2 = 0.95, through an exponential function that suggests a minor varia- tion of the ratio as the surplus increases. The calculated surplus index should therefore be used to better discriminate among highly intensive sys- tems, as it is more variable when fertilization is expected to be much higher than crop removal. Finally, the N removal and the N fertilization can be combined with additional information about the natural availability of soil N mineral- ization and residual mineral N in another com- monly used index: the N apparent recovery. Fig. 2 shows that the N apparent recovery was nega- tively correlated with the nitrogen surplus — more N supplied than taken up by the crop — R 2 = 0.50, and positively correlated to the re- moval-fertilizer ratio R 2 = 0.47. Even when the N surplus was close to zero, the apparent recovery was : 25. This shows that the soils were highly fertile and apparently provided a large share of the N necessary for the crop uptake. 3 . 2 . Temporal patterns of nitrogen in the soil solution Fig. 3 shows the temporal pattern of the nitrate concentration of the soil solution extracted at a depth of 50 cm with porous cups. As it was pointed out above, this depth corresponds to the maximum extent of the active portion of roots in all the examined situations. The nitrate-N concentration was always lower than 15 mg l − 1 in the permanent meadow and the maize system at CD, and in the permanent meadow at CS, while it was about one order of magnitude larger in all the other examined cases. Fig. 2. Significant relationships between some indexes for fertilization efficiency. The calculated surplus is expressed in kg ha − 1 year − 1 of N, the apparent recovery in kg of N uptake per 100 kg of N fertilizer. Fig. 3. Temporal patterns of NO 3 -N in the soil solution extracted by porous cups at a 50 cm depth. The two plots are indicated seperately and the continuous line is the average value. Table 5 Average NO 3 -N concentration of the soil solution extracted at a 50 cm depth and results of the analysis of variance for repeated measures CD PS CS PD 1.0 35.7 1.6 30.5 Field a 2.3 19.8 Field b 52.0 23.0 2.0 18.2 9.7 29.2 Summer 1.3 Winter 38.6 13.1 53.1 1.6 28.4 11.4 41.6 Avg. ANOVA ns + ++ ns Field Season ns ++ ns + ++ ++ ++ ++ Time ++ Field×time + ++ ++ + ns Field×season ns ns ization Cavazza et al., 1986; Grignani and Acutis, 1994. In this trial the effect of spring mineralization on the nitrate concentration was not clearly distinguishable from the increase due to nitrogen fertilizer supply in combination with tillage. When the organic fertilizer was applied in autumn and ploughed in, the soil nitrate concen- tration resulted to be remarkably higher than following autumn applications without ploughing. Examples of the former case were found in PSa 2 , PDa 2 and PDb in winter 1994 – 1995, while an example of the latter was found in PSb for both winters. In the meadows the rather scarce surface N applications did not produce any concentration peaks, although values higher than the average, ranging from 2 to 8 mg l − 1 , were recorded in CSa 1 in winter 1994 – 1995, after a cattle slurry application of about 110 kg N ha − 1 . 3 . 3 . Factors affecting the nitrate concentration of the soil solution The nitrate concentration of the soil solution was affected by the cultivation system in shallow soils CS and PS, as Table 5 reports. The ‘field’ effect should be regarded as the sum of the effects of the crop physiology and the agricultural prac- tices to the crop, including the time and amount of fertilizer applications, the amount of irrigation, tillage, crop duration, etc.. The concentration in the permanent meadow a 1 was smaller than in the maize b at CS, whereas the multiple crop- ping system a 2 involved higher concentrations than maize as a single crop b at PS, which is consistent with what was observed concerning the N fertilizer surplus and efficiency Table 4. The average concentration in winter resulted to be significantly higher than in summer only in PD + 23.9 mg NO 3 -N l − 1 . The opposite trend was observed in CD although the difference was only 0.7 mg NO 3 -N l − 1 , while in the other situations the seasonal differences were not significant, prob- ably also as a consequence of the large variability over the years. Moreover, in three out of four sites the response of the two cultivation systems to the seasonal effect was similar non-significant ‘field × season’ interaction, while in CD the inter- Even though at low concentrations, in CD in the maize-cropped field CDb the soil nitrate content showed temporal and seasonal variations, while in the meadow CDa 1 it was more buffered. The ammonium-N variations were limited over the situations data not shown. The average value was, in all cases, smaller than 2 mg l − 1 and less than 1 of the data exceeded 4 mg l − 1 . A higher NH 4 -N concentration was recorded in the three situations where the nitrate concentration was lower CSa 1 , CDa 1 and CDb, but no signifi- cant relationship was found between the NO 3 -N and NH 4 -N concentrations. These data confirm what was found by Decau 1997 concerning fre- quent high ammonium concentrations in meadows. Despite the fact that the spatial repeatability of the concentrations was reasonably good, with an average CV of 49, the temporal variations of the nitrate concentration were difficult to interpret in all cases. The variability between years was large and no clear seasonal pattern was observed. This occurred also as a consequence of the interactions between the meteorological variability and the inconsistency of agricultural practices over years. Several authors have found that the nitrate concentration of the soil solution increases in spring, as a consequence of the stimulating effect of higher temperatures, rainfall and oxygenation due to tillage, on the soil organic matter mineral- action was significant owing to the higher buffer capacity of the meadow compared to maize, but once again with differences of scarce practical importance. Date by date, the concentration in the two systems was different in all the sites significant ‘time’ effect, and temporal patterns were not parallel significant ‘field × time’ interac- tion. The concentration in shallow soils seemed to be more reactive to crop and tillage effects than deep soils monitored at the same depth, where soil and seasonal effects prevailed. The duration of the crop cover did not necessarily reduce the average soil nitrate concentration, however seasonal varia- tions resulted to be somewhat buffered. On the contrary, several authors have observed that the soil cover during winter reduced the soil nitrate content and smoothed variations over the years e.g. Ru¨egg et al., 1998. Weather conditions pre- sented in Table 2 might help in explaining our results. The crop uptake is limited by cold winter conditions. The summer evapotranspiration is of- ten intense and the severely depleted soil water reservoirs are only partially replenished by au- tumn and winter rainfalls. Therefore, the drainage period is often unpredictable and in winter solute movement within the soil is slow. In both mead- ows examined in this study the soil nitrate content was low and buffered over time, as in the cases examined by Smith et al. 1990, and Grignani and Acutis 1994 for grass and lucerne leys, compared to continuous maize rotation and Ital- ian ryegrass-maize double cropping. However, in none of the examined meadows was the N surplus as high as in the maize. It would therefore be interesting to monitor meadows with higher nitro- gen inputs. To confirm that the soil cover duration was a stabilizing factor on the soil solution concentra- tion, an approximated time series analysis was performed on data, and a serial correlation coeffi- cient was calculated. The shape of the autocorre- lation function, with an exponential decay and damped sine waves, suggested an autoregressive generating process in all cases Box et al., 1994. Table 6 reports the level of autocorrelation for the NO 3 -N concentration in the time series. In all the cases, the longer extent of the autocorrelation period in cultivation systems a compared to sys- tems b, showed that the buffer capacity for min- eral nitrogen was higher where a crop was present during the whole recording period. Instead, in PD the high autocorrelation in the concentration time series, found in both fields, could be due to the buffering capacity of the shallow water table about 1 m deep. In order to test whether various soil and man- agement factors could influence the nitrate con- centration of the soil solution, averaged over the 2-year period, simple correlation coefficients were calculated, and are reported in Table 7. The soil mineral N concentration was correlated with the overall fertilizer input R 2 = 0.33; the total N supply, namely fertilizer and residues of the pre- ceding crop R 2 = 0.42; and the crop uptake R 2 = 0.51. This correlation would suggest that every kg of N input produced an increase in the concentration of 0.07 mg NO 3 -N l − 1 , and that every mg NO 3 -N l − 1 produced an increase in the crop uptake of 2.3 kg ha − 1 year − 1 . However, none of the soil characteristics and the examined fertilizer efficiency indexes proved to be signifi- Table 6 Maximum number of shifted time lags with significant PB 0.05 serial autocorrelation coefficient for NO 3 -N concentra- tion Plot Cultivation system Max significant autocorrelation lag a 1 CS 1 4 2 5 b 1 1 2 2 1 1 a 1 CD 2 1 b 1 1 2 – a 2 1 4 PS 2 5 b 1 1 2 3 1 a 2 5 PD 2 4 b 14 1 12 2 Table 7 Correlations between various variables and the average soil solution concentration a Units Type P Variable Slope Intercept R 2 0.96 Soil physical properties – Clay – – Sand 0.84 – – – cm 0.90 – Root depth – – g cm − 3 0.49 – – Bulk density – mm h − 1 0.41 – K S – – vol. 0.44 – – – Stone content 0.37 Soil chemical properties – C – – N 0.26 – – – pH 0.08 – – – 0.67 – CN – – Humus C 0.78 – – – mg kg − 1 0.68 – Mineralizable N – – meq 100 g − 1 0.07 – CEC – kg ha − 1 year − 1 0.00 Crop 0.22 N uptake -32.0 0.51 kg ha − 1 year − 1 0.31 N removal – – – d 0.79 – Bare soil – – Fertilizer management Total N input fert. + residues kg ha − 1 year − 1 0.01 0.07 -5.70 0.42 kg ha − 1 year − 1 0.02 0.07 -4.48 Fertilizer N min.+ org. 0.33 kg ha − 1 year − 1 0.11 – Ploughed fertilizer – – Fertilizer efficiency Calculated surplus kg ha − 1 0.08 – – – 0.06 – – Removalfertilizer – kgN 100 kgN − 1 0.38 – – – Apparent recovery a The number of cases is 16. cantly correlated to the NO 3 -N soil content. This shows that, in intensive farming, the soil mineral N concentration results from a great number of interacting factors, and simple relationships are not sufficient to predict the risk of N leaching. A multiple regression approach yielded better results, as Table 8 reports. The average nitrate concentration was positively correlated to the plant N uptake as was reported by Brandi-Dohrn et al., 1997, indicating that crops take advantage of a higher N content in easily-available water extracted using porous cups. The relation was inverse for the removal-fertilizer ratio the larger the fertilizer excess, the higher the concentration, as well as for the soil pH and sand content. As the soil biological processes are promoted in sub-al- kaline conditions, the relationship to the soil pH could indicate that a high microbial activity in synthetizing organic compounds is effective in maintaining mineral N concentrations low. The inverse relationship to the sand would suggest that the soil mineral N content is lower where the permeability and drainage capacity are higher. Although these results may serve only as an empirical tool and they do not lead to a mechanis- tic interpretation, they indicate that, in the follow- Table 8 Coefficients of the multiple regression model. Standardized coefficients allow comparisons between variables in different units a b standardiz. Adj. R 2 B coefficient Variables 146.09 Constant 0.18 N uptake 0.58 0.480 N 0.643 − 19.09 − 0.56 removal-fertil izer ratio − 19.60 pH − 0.42 0.739 − 0.43 Sand − 0.32 0.848 a The number of cases is 16. Table 9 Nitrogen leaching as predicted with the LEACHM model in the December 1994–December 1996 period modified from Zavattaro, 1998. Cultivation system NO 3 -N leaching kg ha − 1 1995 1996 CS a 1 4 35 56 b CD a 1 2 b 3 10 PS 61 a 2 37 64 b − 10 PD 21 a 2 b − 4 10 while in 1996 the opposite occurred, but on the medium period these two processes were almost balanced. The variability in leaching amounts be- tween the two years was large the coefficient of variation was only smaller than 100 in CSb and PSb, expecially where capillary rise movements were remarkable. The poor relationship between the average ni- trate concentration of the soil solution and leach- ing R 2 = 0.05, ns, mainly due to the importance of the capillary rise in the examined situations, confirmed that the mineral N content of the soil is only an indicator of potential leaching, and that losses can be evaluated only when both compo- nents are assessed. However, the mean concentra- tion of pollutants in the groundwater recharge is of great interest, as, in the long term, the ground- water assumes the same concentration as its recharge Brandi-Dohrn et al., 1997.

4. Conclusions