Spring barley – grain and straw yield
Grain and straw yield were evaluated by fitting model (1) per rotation over all 6 cycles. Detailed information on model fit, covariance parameters, and test results for the best fitted model can be found in the supplemental material (Table S1, S2). For grain yield, there was a strong main effect of cycle (rotation 1-3: P<0.001), and an interaction effect between N fertilization and cycle (rotation 1,2: P<0.001, rotation 3: P=0.003). The absence of a main effect of N suggests that N affected grain yield differently in different years and had to be partially counteracting. In contrast to grain yield, significant main effects were found for straw yield for both cycle (rotation 1-3: P<0.001) and N (rotation 1-3: P<0.001). However, N also affected straw yield differently among years (cycle x N effect for rotation 1: P=0.006, rotation 2: P=0.036, rotation 3: P=0.045). Straw application had no effect on either trait (except straw yield, rotation 1, P=0.049 for main straw effect), however, barley is grown 2 years after the last straw application.
Since straw application had no effect on barley grain and straw yields, pairwise comparisons of N levels averaged over straw levels are presented (pairwise comparisons as mean impression averaged over all barley years in Fig. 3, comparisons in individual years in Fig. S1, S2). The graphs show the treatment LS-Means with half the critical difference LSD for α = 0.05 as error bars. If the error bars of two mean values do not overlap, a difference between these two treatment means is indicated.
Grain yield of barley was strongly dependent on precipitation. In years with rather above-average precipitation, yields tended to improve with the increase from 40 kg N ha-1 to higher N amounts (1977, 1981, 1993), while in years with below-average precipitation (1985 and especially 1989 with extremely low precipitation in early summer and high temperatures, Fig. 2) the opposite was the case: more than 40 kg ha-1 yr-1 N led to the same or lower grain yield. Rotation 3 with 100% cereals had the lowest yield level, and there were almost consistently no or negative N effects (Fig. S1). On average, more than 40 kg N ha-1 did not result in any barley grain yield gain (Fig. 3), except for the increase from 40 to 160 kg N ha-1 yr-1 in rotation 2.
N amounts affected straw yield more than grain yield, particularly in the two crop rotations with 50% or 75% cereals, respectively (Fig. 3). These rotations almost consistently showed higher straw yields in each year with increasing amounts of N from 40 kg ha-1 yr-1 upward, while rotation 3 showed no N effect in 4 of 6 years (Fig. S2).
Winter rye – grain and straw yield
Grain and straw yield were analyzed per rotation over all 6 cycles (model 1), detailed information on model fit, covariance parameters, and test results for the best fitted model are given in the supplemental material (Table S3, S4).
For grain yield, we identified a strong main effect of cycles (rotation 1-3: P<0.001), N (rotation 1,2: P<0.001, rotation 3: P=0.004), and their interaction effect (rotation 1: P=0.020, rotation 2,3: P<0.001). As for spring barley, straw application had no effect on grain yield, even though spring barley is the preceding crop and the "straw return" treatment supplies barley straw to winter rye.
For straw yield, a strong main effect of cycle (rotation 1-3: P<0.001), N (rotation 1-3: P<0.001), and their interaction effect - only for rotation 2 and 3 - were found (rotation 1: P=0.103, rotation 2,3: P<0.001).
In rotation 1, no effect of straw application was detected; in rotations 2 and 3, this effect was year-dependent (interaction effect cycle x straw in rotation 2: P<0.001, rotation 3: P=0.004), but this effect vanished for all rotations on average over the 6 cycles (no main effect).
Again, since straw application had no or little effect on rye grain and straw yields, pairwise comparisons of N levels averaged over straw levels are presented (as mean impression averaged over all rye years in Fig. 4, comparisons in individual years in Supplemental Fig. S3, S4).
The magnitude of grain and straw yields as well as their differentiation by N levels varied greatly between years. Since in all rye years the precipitation totals were at least average for both vegetation and early summer, the precipitation tends not to be an issue (Fig. 2). Unlike barley, increasing the N amount from 40 to 80 kg ha-1 yr-1 resulted in higher grain yield on average over the cycles, and also from 80 to 120 kg ha-1 yr-1 in rotation 2. A further increase in the amount of N did not bring any benefit. As with barley, N affected straw yield more strongly. For all rotations and all cycles, increasing N from 40 kg ha-1 yr-1 to a higher N level resulted in higher straw yields, and partially the yield even increased up to the highest N level of 160 kg ha-1 yr-1.
Straw amount returned from test crops
In the straw application treatment S11, the harvested straw amount was returned from both test crops. Table 2 shows the straw quantity per cycle for this treatment as the sum of LS-Means in straw yield of both test crops estimated by model (1) depending on the mineral N level. Averaged over the 6 cycles, in rotation 1 N4 produced 1.5 times the amount of straw of N1, in rotation 2 the factor is 1.4, and in rotation 3 about 1.2.
Soil organic carbon content
Unlike the majority of LTEs, annual plot values for SOC in the topsoil are available for our trial. The development of annual treatment means over time is shown in Figure 5. Crop rotation 1 with 50% cereals showed the strongest decline in SOC values, crop rotation 2 showed a less pronounced decline, while SOC values in crop rotation 3 with 100% cereals remained at about the same level.
The treatments receiving straw from test crops had almost consistently higher SOC values than the treatments without straw return. The influence of N fertilization appeared to be less pronounced. For all rotations, the treatment without organic manure since 1937 had the lowest initial SOC value at the beginning of the study period, but became closer to the other treatments without straw return over time.
The four consecutive years of a treatment belonging to a cycle differed both in crops and in the straw application (only in two of the four years), which might have influenced SOC values differently. Therefore, plot values averaged across the years per cycle were used for the following analysis.
Using the 2 straw x 4 nitrogen treatments, the effects of organic manure, mineral N fertilization, and their interaction on SOC were evaluated by fitting model (1) per rotation over all 6 cycles. Detailed information on model fit, covariance parameters, and test results for the best fitted model can be found in the supplemental material (Table S5).
In all rotations there was a main effect of straw application on SOC values (rotation 1: P=0.021, rotation 2: P=0.016, rotation 3: P=0.009), a main effect of cycle (rotation 1-3: P<0.001) and, except rotation 1, also an interaction effect between straw application and cycle (rotation 1: P=0.189, rotation 2: P=0.003, rotation 3: P=0.006). In contrast, there was no effect of tested mineral N rates on SOC, neither as a main effect nor as an interaction effect with straw application or cycle.
Since N had no effect on SOC, but the treatment without any organic manure since 1937 showed a different trend in SOC values (Fig. 5), another model (2) was fitted for the joint analysis of all 10 treatments (see supplemental Table S5 for details). Only the pairwise comparisons of the 4 straw treatments at the same N level (N3 = 120 kg N ha-1 yr-1) were chosen to present the results (Fig. 6). In the first cycle, there was a clear mean difference between the treatment S00# without any organic manure since 1937 and the other treatments in all crop rotations. The differentiation changed in the following cycles, the treatments without organic manure since 1976 became closer in the mean, while treatment S11 with continued organic manure became increasingly different from the others.
Describing the long-term trend of SOC values is often more interesting. With model (3) exponential functions were fitted to estimate SOC trend functions for the four straw treatments (Fig. 7). The LS-Mean values estimated using the exponential functions show the average trend of the SOC values over the study period (6 cycles). The fit of the estimated functions to the observed plot values varied, but the general behavior of SOC values in time, already roughly described in Fig. 5, can be better derived.
Initial SOC values for 1976 ranged from 6.5 to 7.2 g kg-1 soil for both treatments S10 and S11, which were supplied with FYM until 1971. Although also supplied with FYM, the initial value for S10# was slightly lower because this treatment received no potassium until 1971. Treatment S00# without any organic manure since 1937 always had the smallest initial values (4.3 to 5.2). Until 1971, treatments were tested in rotations with 50% cereals, so different crop rotations did not yet play a role. During the study period, the treatment S11 with straw return of the test crops consistently showed the highest SOC values and differed increasingly from the other treatments without organic manure. The latter approached each other on the same level, and the respective change (increase, decrease, stagnation) depended on the SOC initial value. Trend functions of S00# in rotation 1 and 2 as well as S10# in rotation 3 stagnated, these treatments obviously behaved in equilibrium. The rotations showed marked differences in SOC levels and in the magnitude of change in SOC values, obviously there is a negative influence of the non-cereal crops on SOC.
After 6 cycles, relatively stable SOC values can be observed. The comparison of SOC mean values shows that the S11 treatment was superior to all other treatments on average, partially there were also differences between other treatments, especially to S00# with the lowest mean value.
Besides SOC content, the most interesting aspect is the extent of carbon sequestration in the soil and its change due to the different treatments in the three rotations during the study period. Based on the SOC contents estimated by the exponential functions (Fig. 7) at the beginning (cycle=0.5 as initial situation in 1975/76) and at the end of the study period (cycle=6 with the years 1996-1999 as final situation), the SOC stocks (Mg ha-1) in the 0- to 20-cm layer were derived (Table 3). The differentiation of carbon stock values between the straw application treatments as well as their change in time is analogous to SOC content values (Fig. 7).
The magnitude of stored carbon for S00# was always below 18 Mg ha-1 (stagnated in rotations 1 and 2 with values around 13 and 16 Mg ha-1, increased from 15 to 18 Mg ha-1 in rotation 3). For the other treatments, the quantities were between 19 and 22 Mg ha-1 at the beginning, decreased to values between 14 and 17 Mg ha-1 in rotation 1, to values between 17 and 19 Mg ha-1 in rotation 2, and stagnated at the same value or changed only slightly in rotation 3. Treatment S11 had the highest stored carbon quantities at the end of the experimental period.