2.1.3.1 Nitrogen and potassium tracer recovery
Nitrogen and K acquisitions from topsoil (A-Bw horizons), subsoil (Bw-BCw horizons), and saprolite (below BCw horizon) were determined by recovery of 15N for nitrogen and Cs + Rb for potassium. K analogs and 15N recovery were similar from all depths under arid conditions (Figure 2.1-2). Under Mediterranean conditions, on the contrary, tracers for both nutrients were dominantly retrieved from the topsoil (in shoots: 190 times and 37 times higher than from saprolite for N and K, respectively). Under humid conditions, 15N recovery was similar from topsoil and subsoil but was lowest from saprolite (~50% lower than top- and subsoil) (Figure 2.1-2).
While almost no K analogs were retrieved from subsoil under humid conditions, recovery from topsoil and saprolite were similar. Those preferential uptake depths showed no relation to N and K stocks (Figure 2.1-3). Under Mediterranean and humid conditions, recoveries of K analogs relative to 15N recovery was highest from saprolite (Figure 2.1-4), with 10 (Mediterranean) and 5 (humid) times more regained from saprolite than from topsoil. Under arid conditions, on the contrary, relative K analogs recovery was similar from all depths. (Figure 2.1-4).
Figure 2.1-3: Depth profiles of total potassium (K) and nitrogen (N) stocks in 10 cm depth increments in the arid shrubland and Mediterranean coastal matorral, and in 25 cm depth increments in the
humid-temperate forest. Data are presented as means (n = 3) with standard errors. Dashed and dotted lines indicate preferential acquisition depths of the
respective nutrient (blue dotted=
K; red dashed= N) derived from the tracer recoveries (see Fig. 1).
Note the different soil depth scales for the three sites.
Figure 2.1-2: Tracer recovery of N (15N, left) and K (Rb+Cs as K analogs, right) from topsoil, subsoil, and saprolite in shoots (top) and roots (bottom) in the three study sites: arid shrubland, Mediterranean coastal matorral, humid-temperate forest. 15N recovery under forest is additionally presented on a smaller y-axis (inset box). Data are presented as means (n = 8) with standard errors. Significant differences (p<0.05) between depths are indicated with lowercase letters for shoots and with capital letters for roots within sites. Asterisks indicate significant differences between plants within sites.
2.1.3.2 Nutrient distributions
Potassium and N stocks in soil increase with increasing precipitation (Figure 2.1-3). In the Mediterranean and humid ecosystems, N stocks were high in the topsoil and decreased with depth (Figure 2.1-3). The only exception was in the humid-temperate forest in 100-125 cm soil depth, at the interface of subsoil to saprolite (Oeser et al., 2018), where N (and C) stocks were exceptionally high. The N stocks decrease only minimal with depth under arid conditions (Figure 2.1-3).
Contrary to total K stocks, exchangeable K decreased markedly with increasing depth in all ecosystems (Figure 2.1-5). Exchangeable Na, on the contrary, was close to 0 throughout the depth profiles under humid and Mediterranean conditions but increased strongly with depth under arid conditions (Figure 2.1-5). In comparison, the percentage of K saturation of the total cation exchange capacity (CEC) (%K) was 8, 78, and 6 times higher than %Na in the upper 10 cm in the arid, Mediterranean, and humid ecosystems, respectively (Figure 2.1-5).
Figure 2.1-4: Relative K analog recovery per unit N recovery from topsoil, subsoil, and saprolite in the three study sites: arid shrubland, Mediterranean coastal matorral, humid-temperate forest. Relative K analog recovery under shrubland is additionally presented on a smaller y-axis (insert box). Data are presented as means with standard errors. Differences between depths within sites were not significant (p>0.05).
Figure 2.1-5: Vertical distribution of exchangeable K and Na (K, Na) [µmolc g-1], percentage of K and Na to total cation exchange capacity (%K,
%Na) as well as pH in the three study sites: arid shrubland, Mediterranean coastal matorral, humid-temperate forest. Data were taken from Bernhard et al. (2018) and are presented as means of top, mid, and bottom slope positioned soil pits with standard errors. Error bars for pH are omitted for clarity. Note the different soil depth scales for the three sites.
2.1.3.3 Natural 15N abundance
Shoot natural 15N abundance decreased with increasing precipitation, from 3‰ ± 0.6 in G. resinosa in the arid shrubland to -5‰ ± 0.5 in A. araucana in the humid-temperate forest (Figure 2.1-6). In each site, shoots were 15N-depleted compared to topsoil δ15N. The strongest depletion of shoots (Δ >10‰) was observed in the humid ecosystem. Root δ15N were similar between ecosystems and strongly 15N-depleted compared to topsoil δ15N in all ecosystems (Δ 11‰
in the arid shrubland, Δ 4‰ in the Mediterranean coastal matorral, and Δ 9‰ in the humid-temperate forest (Figure 2.1-6)).
Plant C:N ratios of Araucaria araucana in the humid-temperate forest were 2 and 1.7 times higher than for shrubs in the Mediterranean and arid ecosystems, respectively (Figure 2.1-6). Plant N:K ratios were highest in G. resinosa in the arid ecosystem, with 1.7- and 1.5-times higher ratios than of Aristeguietia salvia and Araucaria araucana, respectively (Figure 2.1-6).
Figure 2.1-6: 15N natural abundance (δ15N signatures) of shoots (above the 0-line), roots, and topsoil (left), and plant C:N and N:K ratios (right) in the three study sites: arid shrubland, Mediterranean coastal matorral, humid-temperate forest. Data are presented as means with standard errors. Significant differences (p<0.05) between sites are indicated with capital letters for C:Nplant and with lowercase letters for N:Kplant.
2.1.3.4 Cross-biome analysis
In the cross-biome redundancy analysis (Figure 2.1-7), 15N recovery (Nacq) in plants was strongly related to the relative abundance of bacteria and activity of proteases. Greater bacterial abundance and activity of N cycling related enzymes intensify N mineralization and so, support plant N nutrition. However, 15N recovery in plants did not relate to the relative activity of chitinase (NAG).
Higher precipitation and frequency reduced plant 15N recovery (Figure 2.1-7) because of leaching.
The 15N recovery was higher at lower C:Nsoil (i.e. greater N availability in soil). Higher C:Nplant
(i.e. greater plant N demand) did not increase N uptake (Figure 2.1-7). Similar as for 15N recovery, increasing plant K demand (i.e. higher N:Kplant) did not increase recovery of K analogs. K analog recoveries were higher with greater exchangeable K availability in soil but clay content and precipitation, on the contrary, did not affect K uptake (Figure 2.1-7). Tracer recovery for K and N
did not relate to each other, suggesting that specific factors and processes determine the uptake of both nutrients.
Figure 2.1-7: Cross-biome redundancy analysis (RDA) for 15N and K tracer recovery (Nacq, Kacq), presented as type II scaling (correlation) plot.
Explanatory variables: sum of precipitation (precip) and days with precipitation (frequ) during the 8 month of the experiment, C:Nplant/soil, N:Kplant, exchangeable K (Kexchange), and clay content as well as fungal and bacterial DNA abundance (fungi, bacteria) and protease and chitinase (NAG) activities relative to soil organic carbon. The overall RDA was significant with 999 permutations.