Evapotranspiration measurements and above-ground biomass
Figure 1 shows daily evapotranspiration (ET, mm day−1) of each CC tested before mowing (DOY, day of the year, 184) and at 2, 8, 17 and 25 days after mowing (DOY 190, 196, 205 and 213); bare soil was also included as a reference. Before mowing, ET rates showed significant differences between and within the three groups. CR plants had a mean ET of 8.1 mm day−1, which was lower, compared to the other two groups (10.6 and 18.6 mm day−1 for GR and LE, respectively) and the bare soil control (8.5 mm day−1). On DOY 184, values as high as 9.4 (Glechoma hederacea L., GH) and 9.8 mm day−1 (Trifolium subterraneum L. cv. Denmark, TS) were found (Fig. 1), while ranging around 7 mm day-1, Dichondra repens J.R.Forst. & G.Forst. (DR), Hieracium pilosella L. (HP), and Sagina subulata (Swartz) C. Presl (SS) ET were lower than soil evaporation itself.
Vertical bars represent the daily water use as referred to unit of soil (ET, mm day−1) for the bare soil (yellow) and all the cover crop species as divided into creeping plants (shades of blue), legumes (shades of green) and grasses (shades of orange). Evapotranspiration was measured though a gravimetric method before (i.e. − 4) and at 2, 8, 17 and 25 days after mowing. ET data are mean values ± SE (n = 4).
On the same day, a large ET variation was recorded within the GR group as Festuca arundinacea Schreb. cv. Thor (FA) scored the highest daily ET values (13.4 mm day−1), whereas in Festuca ovina L. cv. Ridu (FO), water loss was reduced by 45% (7.5 mm day−1). Within the 15 CCs, LE registered the highest pre-mowing ET with Trifolium michelianum Savi cv. Bolta (TM) peaking at 22.6 mm day−1. However, within LE, Medicago polymorpha L. cv. Scimitar (MP) showed ET values as low as 12.1 mm day−1 (Fig. 1).
Two days after mowing, all tested CCs recorded ET values lower than 9 mm day−1 (Fig. 1). Moreover, water use reduction among LE ranged between 56% (M. polymorpha, MP) and 73% (T. michelianum, TM), such that T. michelianum (TM, 6.1 mm day−1), Medicago truncatula Gaertn. cv. Paraggio (MT, 5.6 mm day−1) and M. polymorpha (MP, 5.2 mm day−1) registered ET values lower than the bare soil (7.0 mm day−1). Even though registering a consistent ET reduction after mowing, GR retained ET rates slightly higher than bare soil, except for F. ovina (FO), which recorded the lowest at 6.3 mm day−1. Subsequent samplings showed that most of the CCs had a progressive recovery in water use (Fig. 1) and data taken 17 days after mowing confirmed that Lotus corniculatus L. cv. Leo (LC) and all GR fetched pre-mowing ET rates. Medicago lupulina L. cv. Virgo (ML) registered a partial recovery with similar rates (about 13 mm day−1) at 17 and 25 days after the mowing event. F. ovina and all remaining LE stayed below 10 mm day−1 with ET values close to the control until the end of the trial. At 17 days from grass cutting, under a quite high exceeding-the-pot biomass, both G. hederacea (GH) and T. subterraneum (TS) reached ET values as high as 12.0 and 11.4 mm day−1, respectively. On the other hand, D. repens (DR), H. pilosella (HP), and S. subulata (SS) even though with slightly higher ET values than those registered at the beginning of the trial (DOY 184), remained close to the soil evaporation rates until DOY 213.
Aboveground dry clipped biomass at the first mowing date (ADW_MW1, DOY 188) showed large differences among groups, as represented in Table 1. ADW_MW1 within LE was quite variable, as values ranged between 274.3 g m−2 (M. polymorpha, MP) and 750.0 g m−2 (T. michelianum, TM). With a mean value of 565.9 g m−2, LE aboveground biomass was 80% higher than the mean GR ADW_MW1 (110.2 g m-2). F. ovina (FO) scored the lowest value at 48.4 g m−2 among grasses, while within the creeping group, G. hederacea (GH) and T. subterraneum (TS) had biomass development outside the pot edges totalling 89.6 g m−2 and 23.2 g m−2, respectively.
Leaf area index (LAI, m2 m−2) at mowing showed the highest values in LE with LAI peaking at 12.4 (Table 1). Among GR, LAI did not show significant differences, being around 1.2. Concerning CR, LAI was assessed at 0.2 and 0.8 for T. subterraneum (TS) and G. hederacea (GH) respectively, while LAI estimated through photo analysis ranged between 1.3 (D. repens, DR) and 3.6 (T. subterraneum TS).
Evapotranspiration per leaf area unit (ETLEAF) was notably higher in GR, ranging between 7.75 (F. ovina, FO) and 9.22 (Lolium perenne L. cv. Playfast, LP) mm m−2 day−1 (Table 1). In descending order, ETLEAF was the highest in D. repens (DR, 5.46 mm m−2 day−1). Similar ETLEAF was found when comparing some LE and CR species such as M. truncatula (MT, 3.40 mm m−2 day−1), M. lupulina (ML, 4.05 mm m−2 day−1), G. hederacea (GH, 3.68 mm m−2 day−1), H. pilosella (HP, 3.86 mm m-2 day-1) and T. subterraneum (TS, 2.74 mm m−2 day−1). T. michelianum (TM), with 1.81 mm m-2 day-1 scored the lowest ETLEAF of all species (Table 1).
Plotting LAI versus the before-mowing ET yielded a significant quadratic relationship (R2 > 0.76) (Fig. 2a) which helped to distinguish two different data clouds. Till LAI values of about 6, the model was linear, having at its lower end all GR and CR species with the inclusion of M. polymorpha (MP) as a legume, while, at the other end, M. truncatula (MT), L. corniculatus (LC) and M. lupulina (ML) were grouped together. T. michelianum (TM) was isolated from all CCs at 22.56 mm day−1.
Panel (a): quadratic regression of leaf area index (LAI, m2 m−2) vs cover crop evapotranspiration per unit of soil (ET, mm day−1). Each data point is mean value ± SE (n = 4). The quadratic model equation is y = − 0.128x2 + 2.9968x + 5.4716, R2 = 0.76. Panel (b): the quadratic regression between LAI corresponding to the clipped biomass (m2 m−2) and cover crop ET reduction (%). Each data point is mean value ± SE (n = 4). Quadratic model equation is y = − 0.8985x2 + 16.503x + 5.1491, R2 = 0.94.
When regressing the fraction of ET reduction, compared to pre-mowing values vs LAI (Fig. 2b), the same quadratic model achieved a very close fit (R2 = 0.94, p < 0.01). CC grouping was similar to the patterns highlighted for ET, although more accurate predictions were reached at LAI, varying from 0 to 3. A linear ET reduction was shown when LAI removed through trimming ranged between 0 and 6, while thereafter, ET reduction was less than proportionate to the amount of LAI removed. This suggests an LAI of 5–6 as a benchmark, within which it is possible to maximise water use reduction after the trim.
Root growth and soil colonization
Root length density (RLD, cm cm-3) determined for each CC at 0–10 cm and 10–20 cm depth is shown in Table 2. Within the topsoil layer, RLD of Poa pratensis L. cv. Tetris (PP), Festuca rubra L. var. commutata Gaud. cv. Casanova (FRC), and F. arundinacea (FA) peaked at 52.5; 53.7 and 59.0 cm cm−3, respectively, whereas M. polymorpha, (MP), M. truncatula (MT), T. subterraneum (TS) and T. michelianum (TM) did not reach the 10 cm cm−3 threshold (Table 2). L. corniculatus (LC) recorded the highest RLD (29.7 cm cm−3) at 0–10 cm among the LE species while being very close to F. ovina (FO, 30.3 cm cm−3), which had the lowest RLD within the GR group. In the CR group, the highest and lowest RLD values within the top layer were found in G. hederacea (GH) and T. subterraneum (TS), at 26.9 and 7.4 cm cm−3 respectively (Table 2). Looking at the root colonization of the 10–20 cm soil horizon, F. arundinacea maintained the highest RLD (10.7 cm cm-3), followed by L. corniculatus (7.9 cm cm−3). Overall, very low RLD was recorded through this layer in all the remaining CCs.
The highest values of diameter class length (DCL, mm cm−3) for very fine roots (DCL_VF, < 0.075 mm) in the first 10 cm soil were recorded in GR, ranging between 9.75 (F. ovina, FO) and 23.35 (P. pratensis, PP) cm cm−3 (Table 2). All remaining species recorded quite low values, comprised within the 0–4 cm cm−3 range. A similar pattern was observed in the same soil layer for the fine root class (DCL_F, 0.075–0.2 mm), although F. arundinacea (FA)and F. rubra commutata (FRC) scored the highest values (25.74 and 26.10 cm cm−3, respectively). For the same diameter class length, none among LE and CR exceeded the 9 cm cm−3 except for G. hederacea, assessed to be at 16.32 cm cm−3.
A more uniform behaviour among species was found for medium (DCL_M, 0.2–1.0 mm) and coarse (DCL_C, > 1.0 mm) roots although, most notably, L. corniculatus roots showed the highest abundance for both DCL_M (23.08 cm cm−3) and DCL_C (0.54 cm cm−3).
At the 10–20 cm soil depth, GR confirmed the highest values for both very fine and fine roots, with F. arundinacea reaching maximum DCL of 2.269 and 5.215 cm cm-3, respectively (Table 2). L. corniculatus largely outscored any other species for both medium and coarse root diameter (6.173 and 0.037 cm cm−3, respectively), with F. arundinacea ranking second (3.157 and 0.016 cm cm−3, respectively).
The highest root dry weight (RDW, mg cm-3) within the topsoil layer was reached by L. corniculatus (8.7 mg cm−3) and F. arundinacea (7.6 mg cm-3). Notably, such values were significantly higher than those recorded on the remaining species, except for the F. arundinacea vs F. rubra commutata comparison (Table 2). At 10–20 depth, scant variation was recorded in RDW measured in grasses, whereas L. corniculatus held its supremacy within legumes (4.5 mg cm−3). Within the creeping type, D. repens (DR) and G. hederacea (GH) scored RDW values as high as those determined for grass species (namely F. arundinacea , P. pratensis and F. rubra commutata), whereas S. subulata (SS) essentially had no root development.
Soil aggregates and mean weight diameter (MWD)
Table 3 reports the proportional aggregate weight (g kg−1) for both 0–10 and 10–20 cm soil depths. Compared to bare soil, the largest increase in large macroaggregates (LM, > 2000 µm) in the top 10 cm of soil was achieved by L. corniculatus with 461 g kg−1. L. corniculatus differed from the rest of the LE group, whose grand mean (90 g kg−1) was the lowest of the three tested groups. As a legume, T. subterraneum (TS, 122 g kg−1) recorded the lowest values compared to fellow CR species, ranging between 211 (D. repens, DR) and 316 g kg−1 (G. hederacea, GH). GR recorded LM values slightly lower than those of CR, with a mean value of 217 vs 224 g kg-1.
The highest small macroaggregates (sM; 250–2000 µm) in the topsoil layer were found in the bare soil and similarly high values were found in M. polymorpha (MP), M. lupulina (ML), and M. truncatula (MT), while L. perenne (LP), with 298 g kg−1 had the lowest amount. Within the 0–10 cm soil layer, GR scored the lowest mean sM (340 g kg−1), while CR species ranged between 343 (G. hederacea, GH) and 439 (T. subterraneum, TS) g kg−1. The overall range of variation among species within the sM fraction at 0–10 cm was 66% (bare soil vs L. perenne) vs. the 707% variation (L. corniculatusvs T. michelianum,) recorded for the LM fraction (Table 3). Within the upper soil layer, T. michelianum (TM) stands out for the highest values for both microaggregates (m, 53–250 µm) and silt and clay fractions (s + c, < 53 µm) recording 346 and 173 g kg-1, respectively. Even though belonging to the same group, L. corniculatus had the opposite behaviour, recording the lowest values for both m (163 g kg−1) and s + c (63 g kg−1).
At 10–20 cm soil depth, L. corniculatus with 319 g kg−1 LM again outscored all other CCs. A quite homogeneous situation could be spotted within GR; measured LM fractions ranging between 65 and 136 g kg-1 highlighted GR as the most efficient group in LM production in the lower 10–20 cm depth. T. michelianum (TM) is the only one showing an LM value as low as the one of bare soil (36 g kg-1).
Within the 10–20 cm soil layer, a more uniform behaviour was found among species for sM, m and s + c under a range of variation of 64% (bare soil vs L. corniculatus), 56% (F. rubra commutata vs L. corniculatus), and 46% (F. rubra commutata vs bare soil) respectively vs. the 811% variation (L. corniculatus vs bare soil) recorded for the LM fraction (Table 3).
L. corniculatus registered the highest mean weight diameter (MWD, mm) among all CCs in both upper (2.68 mm) and lower (1.98 mm) soil layers (Table 3), while T. michelianum ranked the lowest (0.92 and 0.74 mm, respectively). Within the first 10 cm, GR showed a more homogeneous pattern with an MWD variability of 32% (F. rubra commutata vs F. arundinacea), increasing to 73% in CR (T. subterraneum vs G. hederacea) and 226% in LE (T. michelianum vs LC). Similarly, at 10–20 cm depth, the highest variability was registered in LE (167% for T. michelianum vs L. corniculatus comparison). Conversely, less variability was found within GR (41% for FRC vs L. perenne ) and CR (26% for T. subterraneum vs G. hederacea).
Spearman coefficients (ρ) calculated for the correlations between the aggregate-size fractions, RLD, DCL and RDW are shown in Fig. 3 for the 0–10 cm (A) and 10–20 cm (B) soil depths. For the topsoil layer (Fig. 3a), LM had a close positive correlation with RLD (ρ = + 0.56), DCL_M (ρ = + 0.69) and RDW (ρ = + 0.62). Conversely, sM was negatively correlated with the same diameter class lengths (ρ = − 0.68, − 0.74, and − 0.65, respectively). Overall, a similar pattern was maintained for the 10–20 cm depth, although correlations were in general less tight (Fig. 3b).
Spearman’s correlations for differences in soil aggregate pattern and root traits for both 0–10 cm (a) and 10–20 cm (b) soil depth. Blue colour indicates positive correlation, while red indicates negative correlation.
PCA analysis
The Pearson correlation matrix calculated through the Principal Components Analysis (PCA) (Table S1) for the data pool over the 15 CCs showed that evapotranspiration before mowing (UMW_ET) was not correlated to RLD or any DCL; rather, a very close correlation (r = 0.96) was found vs ADW_MW1. Conversely, ET evaluated 25 days after mowing (MW_ET_25) showed a significant positive correlation with several root growth variables including DCL_C, DCL_M, RDW, and total above-ground dry weight (i.e. the sum of first and second cuts, ADW_TOTAL).
Analysis of the bi-plot (Fig. 4) reporting the positioning of each CC and the direction and magnitude of variation of each variable along F1 and F2 components, enables quite a sharp separation of the three family groups, though with some within-group exceptions.
Principal component analysis for 15 different cover crop species divided as grasses (orange shades), legumes (green shades) and creeping plants (blue shades). Red lines represent active variables like (i) evapotranspiration before (UNMW_ET) and (ii) 25 days after mowing (MW_ET_25), (iii) above-ground dry clipped biomass at first mowing event (ADW_MW1) and (iv) total (ADW_TOTAL); (v) diameter class length for very fine (DCL_VF), (vi) fine (DCL_F), (vii) medium (DCL_ M) (viii) coarse (DCL_C) roots; (ix) root length density (RLD) and (x) root dry weight (RDW). Root traits are mean values of 0–20 cm soil depth.
Within LE, L. corniculatus (LC) clearly isolated itself from the remaining species. L. corniculatus combined a strong and positive correlation with RDW, DCL_C and DCL_M along the F1 component and with UMW_ET and ADW_MW1 along the F2 component. Conversely, the location of M. truncatula (MT), T. michelianum (TM) and M. lupulina (ML) in the biplot was dependent on a close positive correlation along F2 with UMW_ET and ADW_MW1. M. polymorpha (MP) displayed a further distinct behaviour, determined by a strong negative correlation with RDW, DCL_C, DCL_M along the F1 component.
GR grouped in the bottom-right quadrant, except for F. ovina (FO). Once again, though, a different behaviour between F. arundinacea (FA) and F. ovina (FO) was apparent, with the remaining grass species having an intermediate behaviour. F. arundinacea showed a close positive correlation with RLD and RDW along F1, and a negative correlation with DCL_F and DCL_VF along F2 (Fig. 4). Conversely, F. ovina (FO) has a negative correlation with UMW_ET and ADW_MW1 (F2) and, albeit lower in magnitude, with DCL_M, DCL_C and RDW (F1). The three remaining grass species (L. perenne, F. rubra commutata and P. pratense) were essentially grouped together, albeit their behaviour was driven by negative factor scores along the F2 principal components. These CCs set for a negative correlation with UMW_ET and ADW_MW1 and a positive correlation with DCL_VF and DCL_F.
CR had a somewhat more homogeneous behaviour, although G. hederacea (GH) too tended to be isolated in the bi-plot distribution. S. subulata (SS), H. pilosella (HP) and T. subterraneum (TS) were almost insensitive to the variables depicted in F2, whereas their behaviour was largely determined by a negative correlation with some F1 variables, viz., DCL_C, DCL _M and RDW.
Source: Ecology - nature.com
